MemTest86 is one that you burn to a CD and boot with that CD in the tray. From there you run it. From what i see of MemTest is that it is just a app that runs from within Windows. If Windows is accessing the Memory then it cant check it. So that would show why you got now errors.
With MemTest86 you boot to it and it runs with nothign else running. Which means it can test every part of the RAM.
Is this your XP or Vista machine??