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Ive been using bitdefender for two months now, and its very very good. I've had the chance of also using AVG, Avira, Kaspersky, and Norton. AVG is the worst of the bunch in my opinion, when I bought my laptop it had a preinstalled Norton, but when it expired I got AVG free. But after seeing AVG protected laptops still go down due to viruses (specifically 3 of my friends), I took a trial version of Kaspersky and to my best luck I was able to snag a free one year license from them in a promo. After which I used Avira for a while. Kaspersky and Avira are very good antiviruses, but the good thing about avira is that it is has a freeware version. I was able to get BitDefender after it, and right after installing and running my first deep system scan I was able to snag 5 malwares in my system that Avira Premium Security Suite missed, and I've been very much protected. I'm currently using BitDefender Total Security 2009...
and for the question of the other poster as to why people buy security suites instead of going for freeware is that because it does more. in the case of Avira for example:
(F - avira free, P - avira premium, PS - premium security suite)
AntiVir stops all types of viruses - found in F, P, PS
AntiAd/Spyware eliminates ad/spyware - F,P,PS
AntiPhishing proactive protection against phising - F, P, PS
AntiRootkit against hidden rootkit threats - F, P, PS
AntiDrive-by prevents against downloading viruses when surfing - P,PS
AntiBot prevents dangerous bot networks
EmailScannerEnhanced email protection
WebGuard protection against malicious websites -P,PS
RescueSystem create a bootable rescue CD -P,PS
BackupSystem configurable data backup solution - PS
AntiSpam filters out unwanted emails - PS
FireWall protection against hackers -PS
GameMode uninterrupted game play - PS
QuickRemoval eliminate viruses at the push of a button - F,P,PS
NetbookSupport for laptops with low resolution - F,P,PS
ParentalControl block websites unsuitable for children - PS
You can see how much more paid versions have compared to free ones...