Athlon X2 5000+
It's stock speed is 2.6GHz, unfortunately, due to...an incident (bent CPU pins...) it now runs at 800MHz. The funny thing is, Vista actually runs fairly well.
You bent the cpu pins?! You are now probably only seeing one core running constantly at 800mhz explaining why Windows is still running at all. I saw one video on a conference where only the kernel from Vista was loaded. You'll never guess how little memory was needed for that. Try under 40mb!
Maybe this sounds dumb, are you sure its not Cool'n'quiet making it clock down to 800MHz?
Congratulations on the upgrade, those old P3 computers on XP were the good days! *Remembers old Celeron 700MHz*