Upgrading Drupal

Ever since I have upgraded drupal my tabs above my main content (var $tabs) all link to my home page. I cannot figure out why this is. I have installed the devel module and it looks as if the queries are being built correctly. I believe it to be a particular module. Though I am unsure which module it might be. It is a production site, so I don't have a whole lot of leway. I was hoping someone has experienced this before and knows the exact solution. Thanks.

Hi guys,
I know Drupal for some months now and only dealt with Drupal 6 so far.
I was asked to upgrade an old Drupal site which is still in version 4.7.11

It seems that upgrading from 4 to 6 is already a nice exercise for a Drupal beginner. But it's even getting better :
the server where the site was hosted was closing down. I just had the time to copy the files and the database.
And I don't have the admin password, (but I guess that's the smallest problem..)
The site was installed together with other sites in a multi-site installation. I only had access to the folders that concerned the site itself. I don't know if I miss some parts of the site because of that (modules that were in a folder I could not acces..)
And of course the old hoster, who was also the person that set up the drupal site some years ago, is unreachable !
A dream situation :-\

I tried to reinstall the site with Drupal 4.7.11 on my local host. But I only get the homepage all right. The links to all the other pages don't work.
I have met that problem with a Drupal 6 site, too. There the solution was to disable "Clean URLs" before exporting the database. Could that be the solution for a 4.7.11 site, too?
If yes : in my case I can't login as admin and disable the clean URL option. Is there a way to do that in phpMyAdmin directly?

I upgraded to 6.15 from 6.14 and I get this error when I try update.php:

Fatal error: Call to undefined function user_access() in /home/user1/public_html/includes/theme.inc on line 934

When I try to access the site (I'm logged in as admin) the error is:

Fatal error: Call to undefined function user_access() in /home/user1/public_html/includes/menu.inc on line 2476

I want to start a fresh drupal site I've been testing too many modules and am starting to find errors here and there, but I don't want to lose any of my created content so what db tables do I need to save and what do I need to dump to start over fresh but keep my content

I am having several problems with upgrade

parse error module.inc on line 500

Got the following error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.