From a geek level of -1, may I add to Simonta's comprehensive post?
I thoroughly endorse Avast, and in particular its 'boot scan' function which will trap infected Windows system files before any virus/trojan can pick up its skirts and hide itself. NB IF it removes any infected W files you will need to replace them using 'sfc' from your install disc (we are assuming XP?)
Do not attempt to run 2 AV programmes together - they do not normally socialise. Remember that if you simply 'close' one (?AVG?) it will start up again on reboot.