I have a friend with a computer (XP Pro., 768 MB RAM, 40 GB HDD - 90%
full, 1.5 GHz processor) that regularly freezes up (for a couple of
minutes at a time) after about 10-15 minutes and it doesn't always
start up properly - it says he must restart the computer to proceed.

I just started having some strange problems with my Windows XP Professional
(Service Pack 2).  And to get some of the obvious questions out of the way,
I'm all legal, have automatic updates turned on, and run SpySweeper with
AntiVirus regularly.  As a developer, it's not uncommon ...