Hello,
Go to the computer that has this problem, and when you turn it on, and after the computer shows the BIOS flash screens, press the F8 key repeatedly. You should get a menu that asks how to start Windows, in Normal mode, Safe mode, Safemode with networking, and the last one should be command prompt mode.
Start it from either Safe Mode or Safemode with Networking.
If it boots all the way to the login screen, then you either didn't press F8 in time, or you might need to try F6 or F7 keys. I'm 100% sure it is F8 key though. Even try it when it shows the screen with the Moving bar saying "Microsoft Corporation" under it.
When you get to safe mode, You're start menu will most likely work, and you'll be able to fix your problems.
I'm at school right now, but later on when I get home I may be able to help you further.
Best of Luck.