Possible infection not detected by NIS 9 or 10?

Hi, everyone...

I ran into a strange experience the other day on my computer.

It's been a few days, so I'm having a bit of trouble remembering the details!

 

The computer had been sitting idle for a while; logged in.  I tried to use it, but nothing happened.

Tried to shut it down;   but it did not complete the shut down (Windows XP SP3)

Came back to the sign-in screen, but my user name appeared missing. Not a good feeling, since mine was admin, and rest were users!

Tried to shut down again, then

Computer started making a noise, like laughter, or clucking; then a screen appeared briefly saying 'This computer is infected'

Then the computer shut down.

 

After this, it powered up normally, with my ID intact.  Ran NIS 9, which found nothing.

Downloaded a virus checker from AVG technologies, and ran it, it found nothing.

 

I've just upgraded to NIS 10, and run a full system scan, which found nothing but a few tracking cookies.

 

Any ideas?  I don't know if I can trust this computer!

 

Thanks,

 

Gerald