Thanks all of you. Yes, my XP computer have 512 MB or RAM. And i don't upgrade the RAM because my PC don't allow more than 512 MB.
Yes, is true that Norton use a lot of resorces of the system. I see working Norton on PC with 1.5 GB of RAM and make full and quickly scans. But 2 or 3 backdoor trojans make a party with him. Can't clean or delete, can't put in quarantine and always says "Acces denied". So, i don't believe in Symantec products.
What about Panda antivirus? Anybody?