Bugger! The laptop started playing up today - very slow and then it wouldn't boot at all! Those very helpful people at Dell helped it to boot again, but it was still very sluggish when loading programs. I also found some suspicious activity in the start-up routine (various dll files with random letter-strings as their file-name)
Dell are now remotely scanning the PC for nasties!!!
...hhhmm and that random message that popped onto the screen telling me I might need to scan for viruses doesn't bode well either - it wasn't even an AVG message!!! After a bit more research, it looks like the infection is 'Trojan Generic11'.
The tests that Dell carried out lasted most of the day. By 5pm, we were pretty sure most of the nasties had been removed. I'm going to run a load of tests overnight to see if anything else is found.