Try these easy fixes. If they do not work come back and we can disucss the more difficult ones.
Go to device manager make sure all drivers
are working properly.
Type event viewer in search. Then view the event viewer for errors.
Go to an administrator account type cmd, in search.
In cmd type sfc /scannow. It will look for errors and make some changes. If there are errors run it a second or a third time.
If that does not work.
Take your windows 7 DVD. Place it in the drive and boot the computer from the DVD. Then choose repair install. You may have to go to BIOS (check computer manual) and change boot from Hard Drive to boot from DVD/CD.
This should work.