When you open a solution in Visual Studio 2008 (or ealier versions for that matter), it opens all the documents that you did not close before you closed Visual Studio. Is there
Have you tried deleting the .suo file?
It's a hidden file that lives beside your solution (sln) file. suo is "solution user options", and contains your last configuration, such as what tabs you left open the last time you worked on the project, so they open again when you reload the project in Visual Studio.
If you delete it, a new 'blank' suo file will be recreated silently.