I've got an Optiplex GX620. Love it, love it, love it. Bought it for cheap at the Dell outlet store online.
USB keyboard, if that matters. The only software I've discovered that it doesn't like is anything that tries to toggle the keyboard LED's in real-time according to disk activity. No big loss. :-)
To summarize the thread so far:
0. Need a utility to monitor the state of NUMLOCK and switch it back on if it gets turned off, either by keyboard action (butterfingers) or via a rude application.
1. The "standard" AHK script to toggle NUMLOCK doesn't work here, but according to others it doesn't seem to work anywhere.
2. I have other AHK scripts purring along in the background that work fine. I keep everything in autohotkey.ini, but a standalone .exe would be fine.
3. Hacking the registry only sets the state of NUMLOCK at startup. Not my problem, since the Dell BIOS has a nice feature to configure that.