I was unable to add an aspx page to my web site solution.
The error I got from Visual Studio was:

“Could not find file ‘c:Program Files (x86)Microsoft Visual Studio 9.0Common7IDEItemTemplatesCahceWebCsharp1033WebForm.zipWebForm_cb.aspx’.”

This was the solution:

1. Open “Visual Studio 2008 Command Prompt”
2. Run the command: “devenv /installvstemplates”

And that was all to solve my problem!