Have you tried booting it into safe mode? When the computer first starts, you should see an option to hit "ESC" for a boot menu. Then go into that and select to do SAFE MODE with NETWORKING. See if that helps. If it does, then you may have a faulty driver (perhaps video) or, you've gotten some unscrupulous virus.
Before this started happening, was there anything unusual or different that took place, like perhaps an unusually long Windows update?
Lastly, there are many ways to troubleshoot such problems. I strongly suggest visiting the "SevenForums.com", as they've got excellent troubleshooting tutorials as well as very helpful members who are often able to lend useful advice.