I am constantly getting viruses what is the best FREE virus protection for my computer?

[Dunbar Pappy ÏŸÏŸ]

Like perceptions of preventative immunizations recommended for humans, many folks are under the impression that a Windows OS can be protected with a 'shot' of anti-virus: the truth is when using Windows on the Internet, there is no blanket solution from a single source...you must become a security expert on your own, and blend several layers of defenses.
Spybot S&D is a bit long in tooth, and not the best available anti-malware selection.
It is not deployed to detect virus anyway.

'Out-of-the-box' MS Windows absolutely reeks with security problems and bad configurations that average users are unaware of...and so the endless exploits and malware infections.
You must reform your entire Windows strategy and tactics.
"10-Step Security"/PC World; http://www.pcworld.com/article/122500/10...urity.html

Stop using Internet Explorer!
It is the Undisputed World Heavyweight Champ @ passing malicious garbage.

Firefox, beefed up with these effective Firefox Collection items, stops acres of malware seen here in Y! Answers:
https://addons.mozilla.org/en-US/firefox...nbarpappy/
"NoScript" will go a long way preventing "rogue" malware infections.

Turn off 3rd party assets:
Tools> Options> Privacy> top drop menu set to "Firefox will use custom settings..."> Check 'Accept Cookies from sites'
â–º& Un-Check 'Accept 3rd Party cookies'.

Always incorporate a stout firewall: Some freeware is here: http://www.snapfiles.com/Freeware/securi...ewall.html
Look them over and match your needs to skill level.

Use "Sandboxie": http://www.sandboxie.com/ for even better protection.
Core defense applications might include (but not limited to) a robust dedicated anti-virus (I make no recommendations).

And, get this installed: â–º"HOSTS file" (from mvps.org) which blocks evil webpage assets from re-directs which install malware or trashware. (Vista requires special instructions; on-page link)
http://www.mvps.org/winhelp2002/hosts.htm
SpywareBlaster (snoopware prevention and hostfile guardian); http://www.javacoolsoftware.com/spywareblaster.html

If you don't have time or skill for 'updating', have a look @ "Secunia": checks all your 3rd party stuff for updates, and has a one click 'install solution' feature...very handy.
Freeware.
http://secunia.com/vulnerability_scanning/

Most novice users do not like hearing this, but nonetheless, it's true: using computers on the Internet requires you to become a security EXPERT, it's that freekin' simple.
And Windows bears the brunt of the majority of it.