Hi
Have you tried booting into safe mode?
If you can't do that you will probably have to reinstall Windows.
It's hard to tell if it's a hardware issue, but if you can't get the Windows reinstall to complete then that may be it.
since it only takes about a half hour to reinstall Windows I would just go ahead and do it.
If you don't want to lose the data that's on the drive do a repair install and it will replace Windows but not overwrite the drive.
Repair Install - Windows 7 Forums
If you have installed Service Pack 1 and don't have a Windows disk that includes it you will have to remove it first.
Or just go ahead and do a complete reinstall.
How to uninstall Windows 7 Service Pack 1 (SP1)
Since you can't get into...