First, boot from an install CD, go into the Recovery Console, and run "chkdsk /r" (no quotes). This is a hard disk check routine.
Next, if it will boot to Windows, keep the CD in the drive, and go to start > run > "sfc /scannow" (no quotes). This is System File Checker, to verify all of your Windows files.
Finally, if it will boot to Windows, I would suggest doing a System Restore to the last known good point. You will not lose any data since then, just installed programs.
If it won't boot to Windows, go back to the Recovery Console and run "fixmbr" and then try to boot to Windows. If no-go, then back to the Recovery Console, run a "fixboot", and try to boot Windows.
If those don't work, attempt to reinstall Windows and choose the "leave current file system intact" option. It will only rewrite the C:\Windows\ directory, and all of your programs will need to be reinstalled.