I'm really a novice at this but my son has MR 5.0 on a Windows 7 Dell XPX 8700 PC. He ended up with some corrupt files and I was told by a computer tech (he messed with PC for an hour)that I either needed to wipe the hard drive and do a clean install, or try to restore to an earlier image. I had just done an image backup the day before the issues began so the tech wasn't entirely sure the image backup wasn't corrupted too but might be worth a shot.
Once I Optimized the drive I removed the Windows CD and rebooted thinking I would get the option of the MR boot menu but I don't. Guess this is how little I know about how this process works.
One thing you can try is to do a clean install of Windows. Next, install the MR program. Then start MR and create a rescue disc (CD or USB stick). Then you can boot from the rescue media and restore your backup image. Of course, this assumes your image file is not on your original hard drive, but is on another drive.