Thank you both for your help. I´ve tried both solutions, with no luck. Now, the really dreaded BLUE screen appears, with the ceremonial memory dump.
What should I do? Backup of the data on DVD would be simple but I would risk transferring the infected files as well as the normal files.
The thing is that I had Avast and Comodo running when all this happened. Is this a common thing, even with antivirus software running?