Here is a couple easy ones.....Majorgeeks.com is a great place to get all kinds of software, they have a ton of tests and stress programs. Pick through them and try a couple.
Prime95 tests the processor and RAM. It will max the processor and almost all the RAM. When I do an overclock or tweak a RAM setting, I let this run for at least 3 hours. If it passes all tests after 3 hours....I consider it stable, that's me though.
http://majorgeeks.com/Prime95_d4363.html
Case : XION II w/12” green cold cathode
Motherboard : Asus P5N-E 650i @ 1532 FSB
CPU : Intel Q9300 @ 2.875 GHz
CPU Cooler : Cooler Master Hyper TX2
RAM : Corsair XMS2 Pro/pc6400/ 4x1GB/ @800 MHz
Graphics : Evga 9800 GTX @ 760 MHz
PSU : Antec Neo Power 650w / modular cables
OP: Vista Home Pre 64 bit