avast is very famous around here... there is a home edition which is free... avg, which not as many use around here also have a free version... like what TwistedMetal have said...
otherwise etrust antivirus provide 1 year free subscription for Windows users... i kindof like etrust too! but not their antispyware...
i use symantec antivirus coporate edition on my system... it is good and i find it a lot better than norton antivirus... which some of my friends are using...
maybe you can buy security suite which have many additional security modules build in such as firewall, antispyware etc... like zonealarm security suite... which comes with a firewall... a antivirus and antispy tool... in additional also IM security and privacy protection...