There are all kinds of free antivirus protection out there, just Google it and you'll see.
Since I'm all about FREE (these days, who isn't?) I thought I'd tell you what I use and you can look at that, and take a look around the Web yourself and see what you turn up.
By the way, I avoid Norton AntiVirus and McAfee VirusScan like the plague.
Mainly because when I did use them, they missed some fairly common viruses.
And I just don't trust them.

So here is what I use....

Spybot
According to their website, "Spybot - Search & Destroy detects and removes spyware, a relatively new kind of threat not yet covered by common anti-virus applications. Spyware silently tracks your surfing behaviour to create a marketing profile for you that is transmitted without your knowledge to the compilers and sold to advertising companies".
Yes Spybot is free, and has saved me quite a few times.
It's easy to run (I can do it...LOL) and once it's loaded the screens talk you through using it.
Spybot has won the Windows 7 Download 5 Star Award, the Shareware.eu 5 Stars Award, the Chip.eu 5 Stars Award and the Best Privacy Software in 2009 among many others.
You can even follow Spybot on Twitter ( via their lead developer Patrick Kolla)
http://twitter.com/SpybotSD
And I think I mentioned that it's free didn't I?
You can go to their site here http://www.safer-networking.org/en/home/index.html and download it.
There was a "spoof" site awhile back, but this is the link to the real deal.
Bear in mind though that Spybot-S&D is not supposed to replace anti-virus programs, but it does detect some common trojans.
And there may be compatibility issues with Norton Internet Security, but they say that has been fixed in the later versions of Spybot.
And also there may be some problems with Spybot and Internet Explorer 8, but since I use Firefox, I havent run into any of them.

Windows Defender
Yeah I know Windows Defender Antispyware is a Microsoft product and I usually avoid MS like the plague, but so far this has really worked for me.
From their website, "Windows Defender is software that helps protect your computer against pop-ups, slow performance, and security threats caused by spyware and other unwanted software by detecting and removing known spyware from your computer."
And again it is a FREE download (I just like saying that).
You can go here and get it...http://www.microsoft.com/windows/products/winfamily/defender/default.mspx
Windows Defender antivirus protection software is included with all versions of Windows Vista and you can download it for genuine copies of Windows XP Service Pack 2 or later, or Windows Server 2003 Service Pack 1 or later.
From what I understand, if you run into any problems with it, your first two support calls are free as well.

Avast!
Besides having the really cool Pirate sounding name, Avast! Home Edition is free.
From their website, "With over 80 million users of avast!, you can rest assured that you are using one of the most tried and trusted products in Windows security. We support more Windows versions (from Windows 95 to Vista 64bit) than any other anti-malware product. No matter what system you have, avast! has it covered."
And also avast! Home Edition is offered in over 30 language versions.
Avast! has more than 80 million users worldwide, so it can't be too bad huh?
You can read more about it and get the free version here:
http://www.avast.com/

AVG
Now I used to use AVG free antivirus protection and had no problem with it.
But after tangling with the Virtumonde virus, I had to say good-bye to AVG.
Also, for some reason, later versions seemed to start having conflicts with my system and my system updates.
Now you may have a totally different experience, so I'm gonna go ahead and include the link to their site as well:
http://free.avg.com/
By the way, if you've been online for awhile, you may remember AVG under it's original name, Grisoft.
Interestingly, both Avast! and AVG are Czech companies.