To prevent Virus, do not go to any website that have automatic-Download.
if you see a pop-up that advertise you to download or saying you have won something,
do not press "OK". when u download it and your firewall detect nothing, that file may be a trojan. a Trojan file is a file that is undetectable and may cause damage to your computer several days later. to fix trojan files, the only way is to fomat your harddisk. to prevent Trojan, block your internet ads and put firewall to medium.
I strongly sudjest that u should buy AVG anti-virus software plus Comodo Fire Wall.
these 2 software will help u alot. You can download the trail version too.