Try booting from your Windows 7 DVD and select "Next" and then select "Repair My Computer" from the options listed at the bottom of the window. As soon as the hourglass changes back to the normal curser click "Next" and select "Command Prompts" from the menu. In the command prompts window type bootrec.exe /fixmbr and hit enter. If everything goes as it should within a few seconds you'll get a message that it was successfully repaired. Restart your computer without the DVD in it and you should be able to boot back into Windows.