If memtest86 runs w/o errors...

[Monstermile]

Does that mean that there is nothing wrong with my memory?? Is it 100% accurate?? Or could there be other problems with the memory that will not cause errors in memtest but will still cause system crashes and lock-ups.

[nukes]

I don't know why it is, but I've had RAM that ran through memtest86 continually without errors, and it still caused application errors in Windows. When I swapped the stick out it worked fine. If you're getting problems in Windows (even if memtest86 runs properly), try switching to a different stick. If the problems go away then it is most likely your RAM.
It does seem silly though, that a RAM testing program can pass something when its clearly dodgy.

[ragtop]

No test is perfect, but memtest is pretty good. Read all the "fine print" on their website to get some info on what the test does. Also, if you really suspect memory problems, be sure to run all the tests - not just the default ones (there's three tests that aren't included in the default test because they take very long time to run.

[Monstermile]

Well it ran on the default tests for almost 30 hours with 75 passes. Forgot to shut it down before work. No errors. I am now running it under all tests.