Frank,
Thanks for the ideas...
In the mean time, I thought the computer was working properly, so I tried to turn off the ADSL's connection firewall, thinking that might be blocking Windows update...
A second afterwards Avast started warning against "Msdirectx.sys" worm, after telling it to delete it it popped up again a second later. I disconected, re-instated the firewall, and started doing a pre-boot scan...
So I will try all those ideas the moment it finishes (so far it found one file and deleted it).
Many thanks for your help mate.
Do you work at Avast?
Thanks,
Me.