Ok, that is the normal .NET library install location so that's fine. If it had been somewhere else the it would have been more of a problem.
Did you recently install silverlight? It shouldn't cause this by itself though. ANy other programs installed recently?
It seems like it may be a .net based app that's been installed, though why it would be automatically running when your doing nothing else is still a mystery. Or why the app is trying to access a file or registry entry or some other protected windows resouce giving you that error.
You might try running "msconfig" (type it into the start menu search box and hit enter) and look at the "startup" tab. Look for anything suspecious in there. You can disable items or all of them and see if it goes away. THen re enable them one by one til it comes back.