Strange shift-key problem on Dell Dim 9200 keyboard

I recently added a new gfx card (nVidia GeForce 210) and more RAM (to 4 GB) to a three-year old Dell Dimension 9200 (aka XPS 410) and it worked perfectly.
A couple of weeks ago, however, I noticed that neither KB shift key was actually working. So, to type capitals, I needed to use the capslock key. Nevertheless, symbols such as $, % and # remained out of reach unless I could copy them from somewhere.
Thinking that this was a simple keyboard failure, I switched out the old Dell usb KB for a new Logitech Wireless MK250.
Amazingly, although the keyboard seemed to work fine, it displayed exactly the same problem in that neither shift key worked.
Also, the same problem was evident in various of the OSes that I have on this machine such as Windows 7, Windows Vista and Ubuntu 10.4 so it wasn't an exclusively Windows issue.
I posted to the Dell technical forum but have not yet received any useful replies.
I had also seen some references to a shift key problem being related to the sticky-key feature being switched on in Windows. However, after the problem had been removed, I was unable to replicate it by turning on or off the sticky keys.
I have no idea why the problem "went away" but this happened when I rebooted a few times to check if the same shift-key issue existed in all of the other five OSes that I have on this computer. When I got to FreeBSD 8.0, I noticed the problem had disappeared and was equally absent in all other OSes from then on.
This post provides some further comments on the same type of issue while this one points to this shift-key problem being a long-time Dell issue.