The Vista DVD will get you to a Command Prompt but I doubt that a W7 disk would do the same. When you get there, type each of the following, including any spaces, and press Enter after each one:-
expand bootmgr temp
attrib bootmgr -s -r -h
ren temp bootmgr
attrib bootmgr -a +s +r +h
Restart the computer without the DVD in the drive and see if that helped.