I would recommend Prime95 for short term, and Seti for long term
However, saying that, Prime95 will crash after 10 secs on my system (Ahtlon XP 1800+ @ 1686 (147x11.5)) but will run Seti, and everything else i use, perfectly fine for weeks without needing a reboot! Only time i reset is to change hardware!

(borrow cdrom for another machine etc)
As MANY others have said, drop by the DC forum, you will generally get an answer very quickly regarding almost anything, as we have people from all over the world, and therefore someone is almost always lurking there
Confused