I've been using this (Dell SK-8115) USB keyboard with my desktop Windows 7 / Ubuntu computer for years without any problems.
Yesterday, while running Windows 7, I found that pressing the * key on my numeric keypad was not producing any character. Shift-8 would still make a *. The other keys on the keypad seemed to make their characters, and toggling the Num Lock state had no effect on the * key, which did nothing. Typing with other keys seemed to work fine. I restarted the computer, and the * key has worked since then.
What could have caused this?
Edit: Rebooting or booting into Linux did seem to remove the problem. I didn't notice it again for several days. One comment suggested I try removing, cleaning, and re-attaching the key. Since it was working with no sign of unreliability except when it wasn't working at all, I decided to wait to do that until it wasn't working again (because if that was the problem, I wouldn't know if I tried to fix it while it was working). That happened last night, so I investigated more carefully. I hadn't noticed before that the / key on the numeric keyboard also was not working. I removed the * key. It looked clean, and pressing the sensor itself had no apparent effect. I put the key back and it still didn't work, but then I found what was causing this in my case (see my answer below).