That's a weird error, but the system error message would seem to suggest that the CryptoPP project file is corrupt. Is your source clean. or did you make any changes? Could there be an unresolved conflict in the project file?
Ah, dang, I think I know.
Let me guess: You checked out the sources some time ago, opened the 2008 solution with VS2010, compiled, everything worked. Then you wanted to update, so you did an SVN Update on the same folder and tried to open the 2008 solution file again. This time it failed, right?
That's because your VC2010 modified the project files, and there were some changes in the SVN as well; and SVN when it was updating your files, found a conflict that it couldn't resolve automatically. So it left the conflict markers in the file. Unfortunately you didn't notice the conflict and tried opening the project anyway, but it failed because with the SVN conflict markers, the file is not a proper project file anymore.
What you need to do is delete the VC2008 folder from your sources, then do an SVN Update again - it will restore the project files to their SVN state. Then you can open the files with VC2010 (converting and everything).
I'm afraid you'll have to delete the project file everytime you want to do an SVN Update, or else you'll run into this problem over and over again. Or just use VC2008, like the rest of us