I am running a p3 733mhz with 512mb ram with win xp pro sp2 and no problems at all, matter of fact I cant remember the last time my computer crashed or a program froze and I let it run constantly with all kinds of processor chomping apps.
I also run many apps constantly and am not plagued by reboots and other problems - however, my machine will slow to a crawl until I kill FF and then its back up to full speed again. I thought it was something I was running at home but it does the same thing at work.