from my extensive experience i recommend "antivir" by avira its low on system resources and best detection rate,second choice is "NOD32" its also low on system resources and nice protection. all other antiviruses mentioned here like "bitdefender" it detects many positive false also it sometimes can't remove the virus even after detecting it,"kaspersky" takes huge time to scan your disk.mcfee and norton are bloatwares, zonealarm has also become way too heavy in system resources.oh by the way, no antivirus can protect you from visiting shady sites,or if you are using Internet explorer for surfing.