Pages not working

I have loaded powerstore onto my site, changed some of the global settings, made some cosmetic and text changes to the index file, then uploaded the site. At first glance, things look okay on the first page, just need to do some tweaking. If I click on any of the top nav buttons, they go to either "webpage cannot be found" or:

The requested URL /Products_Results.php was not found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
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Apache/2.2.11 (Unix) mod_ssl/2.2.11 OpenSSL/0.9.8i DAV/2 mod_auth_passthrough/2.1 mod_bwlimited/1.4 FrontPage/5.0.2.2635 Server at overheadshelf.com Port 80

I did go into the admin area and entered a test product to see if that would make a product page come up, but still the same result. The only nav button that goes to the 404 is the product button, the rest go to "webpage cannot be found". None of the drop down menus work, they disappear if you try to go down to them.

All the text links on the bottom go to pages, but none of the nav buttons on those pages work either. However, the drop downs on those pages do allow you to click them, they just do the same thing--they either go to a 404 or not found page.

Login and Register buttons do the same.

Any help would be appreciated.

Thanks,
Michelle