Unable to Start Debugging in ASP.NET Applications.
Well, it's happened to my on a few machines so I figured I'd mention it. You're going along, your ASP.NET apps are working fine other than the bugs you've coded in them, and then you reboot one day and poof - as soon as you try to compile you get a “Unable to start Debugging” error message. You can start via Debug-> Start Without Debugging but, well, Debuggers are underrated and life without them sucks.
There are many problems but one that you may not find when you google for it is this - if you've installed Whidbey recently or the 2.0 Framework, you're now probably running ASP.NET under the 2.0 Framework. So if you go to your Administrative Panel -> Internet Information Services -> Web Sites and Right Click on Properties, you should see a tab titles ASP.NET There you should also see a combobox with ASP.NET Version. Switch that to the version your solution is supposed to be working with and your app should work now.
Note that you may get all sorts of other weird errors that make no sense. If you've installed the 2.0 Framework recently, keep an eye out for this is your get some funny behavior in your ASP.NET apps.