i have had it installed and it slowed my system (Compac Pentium III 600 Mhz 256 MB ram) to an absolute CRAWL. my old 80486 was faster but that's another story.
Haha, as bad as all that, eh. I had a Duron 700MHz with 256MB RAM at one point, and it wasn't actually that much different with SP2 installed. I'm surprised you see such a difference with the Pentium, they have a faster FSB and L2 cache than AMD Durons.
EDIT: BTW, you don't need 'Shoot The Messenger' - you can disable the Messenger service manually in Control Panel. If you don't know how to do that, then go for Shoot The Messenger.