I had a PC running Windows 7 that was bluescreening several times a day. I ran PC Doctor, system test, burn in test, and individual memory and hard drive tests. Everything passed at least 2 times. I then, because the bluescreening continued after various driver updates etc ran Memtest86 and it found memory errors in one DIMM in less than 2 minutes...
It would seem like at least the PC Doctor Memory test should have also found this.