Yesterday night I re-formatted my pc (sob... 6 hours starting from scratch

) because I was unable tu install Vista SP1... there were some corrupted sys files.
Now my system il fully patched and at present time noting strange happened

.
The settings when the issue occurred: Firewall in safe mode and Defense in clean (the same that are currently used).
What was I doing when did not respond any more? Simple... switching from an IE tab to another on an informational online paper... and with 55% free memory !!!
CTRL+ALT+DEL not worked at all, so I cannot tell if something was using the cpu highly... but I suppose not because no heavy processes were executing...
(sorry for my poor english

)