I was the local admin on all three tests, and I do have the UDF in the script.

I'm using Radimus' WMIQuery 2.5.

The WMICheck gave me 5.1.2600.2180 on an XP Pro SP2. This was the system that gave me SYS-1234567890 as the SN. It also gave 5.2.3790.1830 for a 2003 Server SP1. That server gave me a blank for the SN. The other machine that gave me a blank reported 1.50.1085.63 as the WMI version. It's a Win2000 SP4.