There may be something else wrong with your PC. You can usually fill your HD completly full and it should still boot.
It's hard to tell what could be wrong without actually checking out your PC, but from your description I would think it's a driver or service issue. LKGC booting sometimes does not roll back new services. is a great resource for boot utilities.
If you have another PC on your network you may be able to connect to the malfunctioning machines computer management MMC (without having to log in to the bad PC) and view the event viewer, that would probably tell you exactly what's going on.