I often find that clicking the normal colsedown sequence - start - Turn off computer - Turn off or restart does not work. The problem is particularly noticeable if the computer has been on for an extended time - several hours. I have tried using Task manager and exactly the same occurs. I habve left the computer overnight having gone through the shutdown pprocess and it is still running in the morning. If I stop it manually using the on/of switch it restarts without problems and appears to run normally. what is causing the shutdown problem ??

There is a good chance, you have some driver problem.
Restart in Safe Mode, and see if Start>Turn Off Computer works there.

It restarts fin in safe mode, but then it does in normal mode unless its been left for a while. Does that take us any further?

WINDOWS XP SHUTDOWN & RESTART TROUBLESHOOTING

http://aumha.org/win5/a/shtdwnxp.htm