Having anti-malware scanners is very important. However, it may not be worth it if the scanner itself make your computer ridiculously slow…The following virus scanners were tested to see which gave the lowest impact on system performance:

Avast! Free 5.0, AVG Anti-Virus 9.0, AVIRA AntiVir Premium 9.0, BitDefender Antivirus 2010, eScan Antivirus 10, ESET NOD32 Antivirus 4.0, F-Secure Anti-Virus 2010, G Data Antivirus 2010, Kaspersky Anti-Virus 2010, Kingsoft Antivirus 9 Plus, McAfee VirusScan Plus 2010, Microsoft Security Essentials 1.0, Normal Antivirus & Anti-Spyware 7.30, Sophos Anti-Virus 9.0.1, Symantec Norton AntiVirus 2010, and TrustPort Antivirus 2010.

The top five virus scanners that least impacted systems were: AVIRA, Kingsoft, F-Secure, Sophos, and Kaspersky. For a list of all the results, as well as a bunch of information on the testing, check out this PDF.