Seems you may have also found the root of your problems already:
Back-tracking the only change I made recently was letting my son start playing Left4Dead 1 & 2
Seems it happened when my oldest son tried to watch a youtube vid.
OK joke time's over.
Haven't dug deep into the bowels of XP in a long time, at least since Win7 Beta. However, back in those XP days I did have the occasional "death bug" as you've described. The solution was to find every entry and reference to the program and remove it, including the offending file itself, and all descendant and clone files. I remember that it was no easy task; one of the offending files had replicated itself into 10 different directories across two drives.
This link below should help get you started (or continued):
http://www.spywareremove.com/removeSystemTool2011.htmlbut like I said it isn't a quick job to fix a deep rooted nasty like you described.
Last virus I had experience with (Win7-coworker's machine) AVG found but couldn't delete. Had to turn off the start up entries, rename the directory, reboot, delete, delete from "add/remove programs" and reboot again, then rescan with AVG (found computer clean). Unfortunately, research after the fact led us to find the origin as being part of a divx player update, and not actually a virus per se, but definitely an unwanted and accidentally installed item.
Anyway, I do hope I was of at least some help.