Without knowing more, it's going to be difficult to diagnose.
Windows 7... 32-bit or 64-bit?
How much memory?
What kind of processor?
Did it just start doing this recently?
Any recent changes to the system, hardware or software?
I had a couple of family members experience something similar. In their cases, they'd both visited sites and clicked links that install a bunch of malware with names like Advanced-System Protector, BlockAndSurf, Boost for Internet Explorer, Browse Safe, RocketTab, Super Optimizer, etc.
Go through your list of programs in the Control Panel and do your best to make sure everything looks legit. If not, uninstall the garbage.
I experienced problems recently that were preventing Windows from booting... it was bad sectors on my hard drive which I was able to repair using the hard drive manufacturer's utility.