Yes that was one of the possibilities I thought of, but I understand that the RAM has been replaced. But it could be a defective batch?

It might be worth verifying the drivers?

http://support.microsoft.com/?kbid=244617&sd=RMVP

And running MemTest 86

I would also disable the sound if it is enabled.

My next move would be to go to another machine in the office and "borrow" the memory, PSU, VDU and video card and all the cables then try those.

If that doesn't work then I would suspect the MoBo.