boot sequence runs up to windows then restarts and repeats.. from the command prompt I managed to get an "out of internal memory" error message when trying to change the directory to c: in DOS.
Memtest is never "done", it just starts again at the beginning. Once it has completed a couple passes with no errors it's safe to say that your memory is fine. After 19 hours I have no doubt that you've completed way more than that.
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I'd give Memtest-86 a few passes to start out with.
Can you get into Safe Mode?
Yeah, I can get into safe mode.. I can't think of what to do here though..
If you can, it means that one of your Startup programs is cheesing you. Add them back in one at a time via msconfig until you find the culprit.
If this doesn't work there are a few more tricks we can try.