http://ubcd.sourceforge.net/
Download and burn it to a CD. Its invaluable !
Its got everything from memory testers, CPU burn in programs, HD recovery / testers, and even booting into a modified Knoppix directly from the CD !
Its one of the best utilities i have ever come across. And being pre-OS can stop a lot of headakes if you want to test your new overclock !
Oh, and considering its booting off the CD, it only takes seconds compared to a floppy disk.
If your a system builder, overclocker, or just generaly like to mess with your PC : Trust me, download this
(copied between hardware and software as it fits into both)