I'm using Windows XP Professional SP2
That is where I experienced the problem until I cleaned the solution. Have you tried right clicking on the solution file in Solution Explorer and selecting "Clean Solution..."? This fixed my issue and I noticed it was after converting a large solution to 2008 from 2005.
If you can't see your solution you can go to Tool -> Options... -> Projects & Solutions -> General and check the "Always show solution" option in order to display it in Solution Explorer.
Chad Scharf
http://www.chadscharf.comDon't forget to click "Mark as Answer" if someone answers your question.