I would try opening a command prompt with administrator privileges and run chkdsk /f . Set it up to run on reboot and then shut down>restart. If you hard drive has many errors it may be failing as Core mentioned.
If not, bring it up in safe mode and run a virus scan, again as Core mentioned could be the issue.
If all that comes out clean then put in the W7 DVD (or boot to recovery if you have a HD recovery partition) and have it run a startup repair. That should attempt to fix any damaged windows system files.
Let us know the results.