Awesome. I just WGA'd my computer (you wouldn't believe how difficult it was to get the wga notify actually installed on here!)
and.. result.
I ran the script, and had the same problem as Jake - wgatray.exe couldn't be deleted.
I terminated the process manually through taskmgr and as expected it respawned.
I then re-ran the script, and it killed it and sucessfully deleted it.
I am thinking that, since this has to be a very fast process, the system had cached the script and therefore ran it faster a second time.
I'm certain that terminating the process with taskmgr had no effect (since it returned of its own accord.)
Alternatively, it might not be so much about how soon after terminating that the delete operation happens, but more about when - e.g. the process could be being monitored every 300msec, and perhaps it's about getting within the time-frame window.
So, I'll modify the script so that it creates a separate "dienow.cmd" script, and runs this two or three times.