This is just a guess, but make sure IIS has the application configured as an ASP.NET 2.X application, and not a 1.X. In IIS expand the list of websites, right-click on your site, click Properties, click on the ASP.NET tab and make sure ASP.NET Version is set to 2.X (where X is multi-digit number that begins with a 0).
If that doesn't work, create a new website in Visual Studio 2005, this time select File System for the Location field. Copy your files into this new folder and add them using Website->Add Existing Item (don't replace the web.config file just yet, as that could be where your problem is). Try running the new site. If it works, try replacing the web.config with the old one, saving a backup of the new one first. If it still works, you're done. If not, then the problem is somewhere in the web.config file, and you need to go through it line by line to find the problem.
Hope that helps.