macho there is a small scene.
there is a kind of virus called zlobDNS i think. wat it does is wen it comes to ur computer it automatically changes ur DNS server address in ur network settings to a bogus DNS which has all da antivirus update servers blocked. wen u try to update u wont b able to. i also got da same thing and i tried so many ways and ultimately i reinstalled XP.
im sure uve got dat scene. jst chk ur TCP/IP properties inside ur network connection's properties!