Thanks for the further info and suggestions.
I'm 99.9% certain there are no viruses (virii?) involved. I'm running NAV2003 with up to date definitions and regular scans with nothing nasty being revealed.
I also used the clock sync utility to reset the time on the occasions it was out on booting up. Must admit I thought it checked the time a bit more regularly than just once a week though.
Over the last few days it seems to have been behaving itself again with the time being correct on booting up, so I think for the time being I'll just keep a wary eye on it. Hopefully it was just "one of those things"
Thanks again.
WF.