If you have the Windows 7 disc, start up the computer and hit F12 to boot off the DVD. Go into the recovery options, choose "command " at the bottom.
Type "C:" without the exclamation marks and hit enter
Type "dir" and hit enter
Make sure the directories look like your C: drive, there should be the Users, Windows, Program File folders etc. If it is not right, try D:, E:, etc.
Once you know you are in the right drive, type "chkdsk /r" and hit enter. It will do a full check disc which will take about an hour.
After it is done, type "bootrec.exe /fixboot" and hit enter then "bootrec.exe /fixmbr" and hit enter. Reboot and try to first.
If you don't have the DVD, try to hit F8 and choose "repair your computer", then follow the steps above