Upgrading Drupal

I just update from 6.1 to 6.4. All the add-on themes and modules are gone. It's like a brand new installation. Luckily, the content data remain. But I have to redo all my work to customize my site.

What's the wrong with my update?

I am practicing the upgrade from 5-x.10 to 6-x.4 on a test server. I think I am following all the rules, turning off all non-core modules, switching to Garland theme, but each time I get the following errors:

user warning: Table 'cache_block' already exists query: CREATE TABLE cache_block ( `cid` VARCHAR(255) NOT NULL DEFAULT '', `data` LONGBLOB DEFAULT NULL, `expire` INT NOT NULL DEFAULT 0, `created` INT NOT NULL DEFAULT 0, `headers` TEXT DEFAULT NULL, `serialized` SMALLINT NOT NULL DEFAULT 0, PRIMARY KEY (cid), INDEX expire (expire) ) /*!40100 DEFAULT CHARACTER SET UTF8 */ in /home/chrispel/public_html/crsny.net/drupal/includes/database.inc on line 514.

user warning: Table 'cache_form' already exists query: CREATE TABLE cache_form ( cid varchar(255) NOT NULL default '', data longblob, expire int NOT NULL default '0', created int NOT NULL default '0', headers text, serialized int(1) NOT NULL default '0', PRIMARY KEY (cid), INDEX expire (expire) ) /*!40100 DEFAULT CHARACTER SET UTF8 */ in /home/chrispel/public_html/crsny.net/drupal/modules/system/system.install on line 1403.

Hello geeks ;-)

I just uploaded all files from the 6.4 update and used this guide:
http://learnbythedrop.com/drop/70

But somehow I got away from the windows, where I was logged in as Admin
and user 1. So I tried to use: http://www.hvrforum.dk/?q=user and logged in there.
Afterwards I try to access update.php but gets this error:

After uploading the latest version of drupal and a few modules, I ran the database update thinige (backed up my database, just as it said i should do), now the site is riddled with weird errors.... which I guess I can deal with.. but I get the following when trying to access my modules page and 80% of the time when trying to add content... it also appears sporadically when i browse the site it seems....

Fatal error: Allowed memory size of 33554432 bytes exhausted (tried to allocate 923295 bytes) in /home/mysite/public_html/mysite.com/includes/database.mysqli.inc on line 303

When I put my site in maintenance mode, the screen drupal show to other user is the default one: blue/white with the drupal logo. How can I make this to look like my site in my color scheme and my logo?

I´ve recently installed logintoboggan module in version 6.4, and the page didn´t reloaded... when it finally did, it showed LOTS OF ERRORS. (I´ve been using logintobbogan since 4.7, so, don´t know what could have go wrong).
So, I´ve decided to delete it.

But since then, all modules that are related to User access/authentication breaks my site.
I´ve repair the tables, reinstalled (using devel), the system module... but nothing happens...
What could it be wrong? I´m really desperate here.

Thanks for your help and advice!!!

Rosamunda