This week I needed a particular VisualStudio 2005 project template called "ASP.NET AJAX-enabled Web site" and I could not get it to show up.  Here is how I fixed it.  You might also find these instructions interesting if you are simply trying to use Ajax in VS2005. 

• Uninstalled VS2005 (and VS2008 in my case)
• Uninstalled everything VisualStudio related.
• Installed VisualStudio 2005
• Installed VS2005 SP1
• Installed ASP.NET AJAX Extensions 1.0
  

Whalla!  VisualStudio 2005 is now Ajax enabled!

As a note, I tried installing all of these things in all different orders and it never showed up.  I had to uninstall to fix the problem.  So if you are having a lack of Ajax, give it a try.