Nobody mentioned avast........ its an excellent antivirus as far as system speed is concerned...... I think this one is the only antivirus which gives you the capability of boot sector scanning. Thus even if you pc is infected badly, formatting is not the last option with avast. I once got the situation when my brother downloaded some bullsh** from some torrent sites and that infected my pc badly. That time i dont wanna format my pc and AVG(which was installed that time) was unable to remove the virus. Then i installed avast and it swiped away the virus from my disk. Some little bit registery tweaking with hijack this and tune up utilities and my windows is healthy again. One more idea to be secure is that use 64-bit windows, if you can. i have been using a 64-bit vista for past 6 months without an antivirus program, not even a single sign of a threat.
The best anti virus is NOD32 but I recommend Comodo.com which is free and has anti spyware, firewall and anti virus.
Best antivirus is NOD32 or Kaspersky. Both are always in the top from independent antivirus tests! And no, NOT avast... (or you better dont install a antivirus )
Independent anti virus tests: http://www.av-test.org/ Also check this: http://www.virusbtn.com/news/2008/09_02
Avira is respected here, try Avira AntiVir Personal 8.2.0.337. http://www.filefactory.com/file/a041e0h/n/antivir_workstation_winu_en_h_EXE Code (markup):
Why download your ".exe" instead to FREE downhload from the original website and take the risk you change the code with a trojan/malware inside FREE download from the real source: http://www.free-av.com/en/products/1/avira_antivir_personal__free_antivirus.html
Trend Micro Internet security is also pretty good one. Im using them from last 2 years, so far they are on top.