As Marcus Kramer says, but also Also, check the 'ASP.NET version' configured against the Website. You'll find this under the ASP.NET tab of the Web Site Properties. It should be 2.0.50727 or 4.0.30319 but could well be incorrectly set to 1.1.4322 which would have this effect.
Also, have a look the
Error: Unable to Start Debugging on the Web Server[
^]
entry on MSDN. It could save you a lot of time over trying one solution at a time and coming back to repost...