drupal 7 install.php won't run

is drupal installed in a subdirectory?
Where is the page coming from that is currently showing? obviously from the host, but is it included in your files? index.html perhaps? if so, remove it?

my guess is that any script you add won't work, have you tested? If no script will work, it's not a drupal specific issue.

I know it's bad form to follow up on one's own post but hopefully people won't mind as I think I've solved my problem!

Quite simply, I think I was timing out. I edited my php.ini file to increase the timeout to 120 seconds and I've managed to create a couple of Drupal sites! In case anyone's interested, here's my (current) php.ini file showing the changed sections with the changes I've made prefixed by commented rows starting with GAPS (my initials) describing what changes I've made.

Good luck and apologies for the length!

Changes in my php.ini file:

...

;;;;;;;;;;;;;;;;;;;
; Resource Limits ;
;;;;;;;;;;;;;;;;;;;

; GAPS 9/1/2012 Increased next two time limits from 30 and 60 respectively to 120
max_execution_time = 120 ; Maximum execution time of each script, in seconds
max_input_time = 120 ; Maximum amount of time each script may spend parsing request data
;max_input_nesting_level = 64 ; Maximum input variable nesting level
memory_limit = 128M ; Maximum amount of memory a script may consume (128MB)

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Error handling and logging ;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

; error_reporting is a bit-field. Or each number up to get desired error
; reporting level
; E_ALL - All errors and warnings (doesn't include E_STRICT)
; E_ERROR - fatal run-time errors
; E_RECOVERABLE_ERROR - almost fatal run-time errors
; E_WARNING - run-time warnings (non-fatal errors)
; E_PARSE - compile-time parse errors
; E_NOTICE - run-time notices (these are warnings which often result
; from a bug in your code, but it's possible that it was
; intentional (e.g., using an uninitialized variable and
; relying on the fact it's automatically initialized to an
; empty string)
; E_STRICT - run-time notices, enable to have PHP suggest changes
; to your code which will ensure the best interoperability
; and forward compatibility of your code
; E_CORE_ERROR - fatal errors that occur during PHP's initial startup
; E_CORE_WARNING - warnings (non-fatal errors) that occur during PHP's
; initial startup
; E_COMPILE_ERROR - fatal compile-time errors
; E_COMPILE_WARNING - compile-time warnings (non-fatal errors)
; E_USER_ERROR - user-generated error message
; E_USER_WARNING - user-generated warning message
; E_USER_NOTICE - user-generated notice message
;
; Examples:
;
; - Show all errors, except for notices and coding standards warnings
;
;error_reporting = E_ALL & ~E_NOTICE
;
; - Show all errors, except for notices
;
;error_reporting = E_ALL & ~E_NOTICE | E_STRICT
;
; - Show only errors
;
;error_reporting = E_COMPILE_ERROR|E_RECOVERABLE_ERROR|E_ERROR|E_CORE_ERROR
;
; - Show all errors, except coding standards warnings
;
error_reporting = E_ALL

; Print out errors (as a part of the output). For production web sites,
; you're strongly encouraged to turn this feature off, and use error logging
; instead (see below). Keeping display_errors enabled on a production web site
; may reveal security information to end users, such as file paths on your Web
; server, your database schema or other information.
;
; possible values for display_errors:
;
; Off - Do not display any errors
; stderr - Display errors to STDERR (affects only CGI/CLI binaries!)
; On or stdout - Display errors to STDOUT (default)
;
; To output errors to STDERR with CGI/CLI:
;display_errors = "stderr"
;
; Default
;
; GAPS 8/1/2012, changed from Off to On
display_errors = On

; Even when display_errors is on, errors that occur during PHP's startup
; sequence are not displayed. It's strongly recommended to keep
; display_startup_errors off, except for when debugging.
; GAPS 8/1/2012, changed from Off to On
display_startup_errors = On

; Log errors into a log file (server-specific log, stderr, or error_log (below))
; As stated above, you're strongly advised to use error logging in place of
; error displaying on production web sites.

; Set maximum length of log_errors. In error_log information about the source is
; added. The default is 1024 and 0 allows to not apply any maximum length at all.
log_errors_max_len = 1024

; Do not log repeated messages. Repeated errors must occur in same file on same
; line unless ignore_repeated_source is set true.
ignore_repeated_errors = Off

; Ignore source of message when ignoring repeated messages. When this setting
; is On you will not log errors with repeated messages from different files or
; source lines.
ignore_repeated_source = Off

; If this parameter is set to Off, then memory leaks will not be shown (on
; stdout or in the log). This has only effect in a debug compile, and if
; error reporting includes E_WARNING in the allowed list
report_memleaks = On

;report_zend_debug = 0

; Store the last error/warning message in $php_errormsg (boolean).
track_errors = Off

; Turn off normal error reporting and emit XML-RPC error XML
;xmlrpc_errors = 0
; An XML-RPC faultCode
;xmlrpc_error_number = 0

; Disable the inclusion of HTML tags in error messages.
; Note: Never use this feature for production boxes.
; GAPS 8/1/2012, was commented out, enabled and changed from Off to On
html_errors = On

; If html_errors is set On PHP produces clickable error messages that direct
; to a page describing the error or function causing the error in detail.
; You can download a copy of the PHP manual from http://www.php.net/docs.php
; and change docref_root to the base URL of your local copy including the
; leading '/'. You must also specify the file extension being used including
; the dot.
; Note: Never use this feature for production boxes.
;docref_root = "/phpmanual/"
;docref_ext = .html

; String to output before an error message.
;error_prepend_string = ""

; String to output after an error message.
;error_append_string = ""

; Log errors to specified file.
;error_log = filename

; Log errors to syslog (Event Log on NT, not valid in Windows 95).
;error_log = syslog

; **** Note in the following section I created a new temporary directory for convenience ****

;;;;;;;;;;;;;;;;;;
; Fopen wrappers ;
;;;;;;;;;;;;;;;;;;

; Whether to allow the treatment of URLs (like http:// or ftp://) as files.
allow_url_fopen = On

; Whether to allow include/require to open URLs (like http:// or ftp://) as files.
allow_url_include = Off

; Define the anonymous ftp password (your email address)
;from="john@doe.com"

; Define the User-Agent string
; user_agent="PHP"

; Default timeout for socket based streams (seconds)
default_socket_timeout = 60
; GAPS 8/1/2012 Added following line and commented out one after
upload_tmp_dir = "C:\Program Files\Apache Software Foundation\Apache2.2\Temp"
; upload_tmp_dir="C:\Windows\Temp"
; GAPS 9/1/2012 Added following line and commented out one after
session.save_path="C:\Program Files\Apache Software Foundation\Apache2.2\Temp"
; session.save_path="C:\Windows\Temp"
log_errors=On
; GAPS 8/1/2012 Added following line and commented out one after:
error_log="C:\Program Files\Apache Software Foundation\Apache2.2\Temp\php-errors.log"
; error_log="C:\Windows\Temp"
extension_dir="C:\Program Files\PHP\ext"

...

; **** I think the following went wrong as I reinstalled PHP with more extensions, you may not have this ****

[PHP_MYSQLI]
extension=php_mysqli.dll
; GAPS 9/1/2012 Commented out next two lines from my original file
; [PHP_PDO]
; extension=php_pdo.dll
[PHP_PDO_MYSQL]
extension=php_pdo_mysql.dll
[PHP_XMLRPC]
extension=php_xmlrpc.dll