About two months ago I upgraded my system from Windows XP to Windows 7 64-bit, and at the same time from 4G to 8G RAM. As always happens, I was amazed how much faster a new Windows installation was than the old one on the same hardware -- it is insidious how "Windows Decay" chips away at performance.
About a week ago, the system started behaving badly -- IE crashing, Thunderbird crashing, and starting yesterday, the whole thing blue-screening. After wasting a lot of time trying to figure out "what software was updated recently", I started to suspect memory errors. So I ran the Windows memory test program that shows up on the boot screen -- nothing.
After more dorking around, I downloaded and ran MemTest86+ (www.memtest.org), burned it to a USB drive, and ran it. It immediately found thousands of memory errors; by trying various combinations and moving modules from slot to slot, I was able to identify the bad modules. I had bought Crucial's top of the line (Ballistix Tracer LED) from Newegg; the Crucial folks immediately shipped out a replacement.
Given how many errors MemTest found, its amazing that the Windows test found nothing.
Thumbs up for MemTest86+ and Crucial customer service. Thumbs down for Windows Memory Test.