Actually for the most recent particle forge I think you need .net 3.5. But windows 7 should have it already...
The easiest way I've found to double check that you have it is to put this into internet explorer (yes it has to be IE unfortunately).
javascript:alert(navigator.userAgent)
If you see a .NET CLR 3.5... come up anywhere after putting that in your address bar you have it and I guess it is a different problem.