Further to my previous posts, I have ended up replacing the CPU: all the problems went away permanently... even though I replaced the following compoents - HDD, Video, Motherboard and RAM - the computer was still tetchy. Changing the video card to an ATi stopped the Windows Update problem, but only changing the CPU (from X2 dual-core to a 64-bix Athlon single-core) did the trick of fixing the random restarts and lock-ups. Now, how weird is that?