Stop external USB keyboard repeating key presses on Vista
Ahh... I've been through this with my Latitude D620 and my D630-derived Precision. In my case it is due to the PR01X port replicator and having the USB keyboard and USB Mouse (it judders) connected directly to the replicator. As a workaround plug the keyboard+mouse straight in to the USB on the side of the laptop.
For reference both the A02 versions of the replicator have this issue but an A00 one I have does not.
I hope that helps!
Is it a wireless external keyboard? I have the same problem with my Logitech wireless external keyboard and it's because it loses wireless connectivity with the base randomly.
Refer to the following link: http://forums.logitech.com/logitech/board/message?board.id=software&message.id=3745
One workaround for you to try is to tweak the key repeat delay and repeat rate in Control Panel > Keyboard, or disable key repeat entirely.
I'm not saying it was happening in your case as you seem to have independently verified the issue, just mentioning it here as a pointer for future people searching with a similar problem.
The only times I've ran into this same issue, it was because something was leaning on the delete button on one of the keyboards and the user didn't realize it - seriously. I've seen the same problem with the Enter key on external keyboards because of similar issues and due to it's location on the numeric keypad. The delete issue is more common with people on laptops, again due to key positioning, especially if a lead or something gets caught when closing a laptop lid in an external keyboard, monitor, mouse set up.
Despite it being such an obvious cause, you'd be surprised at the calibre of people that I've seen it catch out...