Basically, if Visual Studio has been referencing an assembly/DLL which no more exists in the system(one reason is while uninstalling someother application), this issue can raise.
Solution: Remove the extinct reference from the solution and things should turn fine.
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Re: SP1 causing "Adding reference to System.Web.dll ..."
Jul 12, 2007 02:56 AM|prbabu|LINK
Basically, if Visual Studio has been referencing an assembly/DLL which no more exists in the system(one reason is while uninstalling someother application), this issue can raise.
Solution: Remove the extinct reference from the solution and things should turn fine.