Well my 64 bit Win7 computer with the RealFlight installed on it isn't quite as hot a machine as yours, but I installed the RealFlight 5.0 which I bought just before the release of the 5.5 and did the online upgrade to the version 5.5 without a hitch and it runs fine.

Suspect you have other issues but my first action would be to call the KnifeEdge guys and see what suggestions they may have, try booting into the safe mode as suggested and remove the software or other than that IIRC, the Win7 install disc will offer you an option to repair an install if you boot it up?

If you cannot even boot into the safe mode, then it kind of looks to me like you've got a corrupted boot sector on your HD but it's been a long time since I really played with my computers unless I absolutely have to....