Desktop

In the BIOS make sure that (in the USB section) "Legacy" is marked "Enable".
If Ok then it might be a few odd keyboards being up the creek. Get hold of a good keyboard and do a test with all the computers.
Another thing, make sure you have updated the BIOS of the computers in question.
One more thing, have a look inside the computer cases, the usb's are sometimes on a daugterboard, make sure these have not come off or are loose, take out and replace if necessary.
Hope this helps you out.

If these systems are running the Intel Chipset, I have came across the Intel Chipset Driver causing the same problem (it just completely disables usb support in windows for some reason). If there is a way that you can get the system up and going, try removing the Intel chipset driver and see if that helps/solves your problem. This usually only happend to me after a new install and installing the driver, but since Dell's come w/ a pre-loaded OS, the drivers are not installed until after the system goes through it's SysPrep process, and during that time it is using the windows generic usb drivers.

Hey Snuffy, did you ever figure out what was going on with those Optiplex boxes? I ran into the same thing with other dell models and found that putting the keyboard and mouse on different channels it would resolve the problem. However if the user had a non dell keyboard such as an ergo keyboard then all bets are off.