If you have both Visual Studio 2008 and Visual Studio 2010 installed on your machine, and you uninstall one of them, you loose the file associations for Visual Studio.
To restore the File associations back, follow these steps:
Open Visual Studio > Go to Tools > Options. When the Options dialog opens, looks at the lower left corner for a ‘Show all Settings’ checkbox.
Select this checkbox > Go to General Page of Environment and click the ‘Restore File Associations’ button. Click OK when done.
Here’s a screenshot
Your file associations should be restored now.