Perhaps there still remains a memory problem. (2 incompatible sticks? memory too slow for computer?) When the memory is working, WCG should then start sending good workunits.

Excessive overclocking can also cause tossing bad workunits.

Run a good memory test. memtest86+ comes to mind. Read about it here. Links to get it is also in there:

http://en.wikipedia.org/wiki/Memtest86

I had two sets that I had to take back before I found a third that worked perfectly.

There should be no need to reinstall WCG if it is due to bad memory.

WCG pushes the memory and computer fairly well. Even things like prime95 may not catch it. Ordinary programs may still run because they only need part of the memory.