11.00am - Thanks to two very helpful people at Dell - one a specialist hardware engineer and one a specialist in software - it looks like the problem is solved - see yesterday's blog.
I don't think any of us are sure WHY the problem ocurred in the first place, but the consensus seems to be a XP shutdown that didn't shut down properly!
A quick look under 'advanced' in 'My Computer' revealed an option to EXCLUDE certain programs from the check that XP does under 'Data Execution Protection'. In my case it was 'explorer.exe' that was complaining..so we excluded it.
Obviously, there is a reason why 'explorer.exe' objected...but a scan for nasties didn't reveal anything err...nasty!
Phew...for one moment, I had a feeling I was going to be reinstalling XP today!