Prime95 and MBM (Score 1)

Prime95 is an invaluable tool for stressing the CPU and Memory of a system. I have found that a system may pass Memtest86, but will crash under Prime. Specifically the new version lets you stress the CPU, RAM, or a mix of both. Alongside that I also find Motherboard Monitor a great asset to any PC diagnostic kit. The ability to monitor temperatures and voltage levels is extrememly important. A weak +12v rail on a newer P4, Athlon XP system is indicative of system instability and most likely the cause of many problems. Also keep in mind heat is electronics worse enemy. I cannot tell you how many times I have found dust caked in the CPU heatsink and fan causing overheating, leading to system crashes or a dead Proc. Not the end all be all of diagnostic software, but a good start. I find the free alternatives better than any packaged software you can buy.