Do you have enough diskspace?
Did you check screensaver, it should be off or at least not returning to user selection during conversion.
One simple thing to try - take out your RAM chips, then blow out dust from their sockets, put them back in making sure they are properly in.
Then clean CPU heatsink, blow out all dust in it, clean also all cooling fans.
And last resort - use HiJackThis to check your system.