I think i may have found the solution. I googled the query and found many with the same problem. Check these links, they may work for others.
It involves going into the Administrative Tools, then Services and changing one (or more) of the MCE services to 'Local System Account', then restart the PC. So far so good, I've restarted the PC 3 times so far with no loss of signal errors and MCE seems to be wroking a treat again. (contented sigh).
Although this link was for a HP/Compaq solution it seemed to work for me (running a Chillblast Fusion Juggernaut with Vista Home Premium)
See also these other links