Once it's booted (assuming that you can again) check the event viewer under Administrative Tools in the Control Panel to see what it was hanging up on. You could also check the bootlog. Once in to Windows, I'd also suggest running Check Disk to fix any file system errors.
It's highly unlikely that a simply monitor change would cause a boot issue. You might want to look at a loose video card as a potential problem and double check all your add in cards while your at it. Moving around PC's has a way of knocking things loose.
The fact that the PC booted past POST leads me to conclude that the keyboard is a non-issue. Generally speaking a PC will not POST without a keyboard.
Some info from MS:
http://support.microsoft.com/default...b;en-us;314503
[ 11-16-2005, 06:42 PM: Message edited by: Zebodog ]