There could be a few reasons for this. Your .NET Framework installation could be hosed. If this is the case, you'll want to uninstall and then reinstall the framework (you'll need .NET 3.5 for the latest version of AspDotNetStorefront). If you don't want to uninstall and reinstall you may want to check out this social MSDN link (there are several .NET verification and cleanup tools available, though I've never used any myself...use at your own risk).
You may also want to verify that you don't have a firewall or anti-virus program blocking access to the files from the application. I've seen several instances where AVG and Panda (just to name a couple) have prevented a recompile until they were turned off or rules were created to allow access to the necessary files.
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