Boot up with your windows dvd, make sure to enable bios to boot cd/dvd drive first. When the screen appears asking to install, on the bottom left of the screen click on the repair option. You can try using system restore, or check startup problems, if any problems found windows will be repaired via the dvd. Try startup fix first.
If eventually you want try command prompt option which is also available, if there other options fail.
In command prompt type :
sfc /scannow
This will check windows integrity and if any errors are found it will attempt repairs automatically.