Just a note: I dug one of oldest towers out of the 'inventory' last week, blew off a pound of dirt and resurected it to crunch numbers for BOINC-WorldCommunityGrid.org (I have other posts). It was the 9th 'oldie' i've put on line for this effort.
After a few burps and gurgles, XP came to life, made a dozen setting adjustments and loaded the pgms for BOINC.
It's been operating 3 days now, and it's just as fast at delivering finished work units as the 3 x 566Mhz eMachines, and another 733Mhz tower.
Old don't mean junk ! Good luck on finding your parts.