Boot the Windows 7 DVD Repair console or Repair CD, accept any offered repair and see if it starts.
If not, boot back into Repair console, click through to Recovery Tools list to run Startup Repair up to 3 separate times with reboots.
In your first post, you say that you are trying to repair Windows 7, but later you say you have XP but are attempting to install Windows 7. Which is it?