Avast, 'cause it's free, reliable and 'cause I'm a beta tester xD
Anyway, with Avast free antivirus you will have the possibility to scan for outdated software and update them directly from the avast gui, a secure browser provided by avast and made from chromium sources, the possibility to save all your passwords and encrypt them and, last but not least, a sandbox where you can run every program you believe it's faulty or dangerous.

Oh, almost forgot, you can run a boot-time scan, which means avast will reboot your computer and run under DOS in order to remove even the worse threat (virus, spyware, rootkit, trojan, malware, worm, grime).
Keep it simple, join avast! ^_^
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