virus/spyware/malware etc protection?

My mother had a subscription to McAfee for all of the above. It's expired and we can't afford to replace it. Are there any free programs to keep our PC in good shape? I'm actually a Mac user at heart so I know little about all this stuff. TIA!

AVG is a free anti-virus program

Ad-Aware has a lite/free version for malware

Spybot Search & Destroy is free and takes care of spyware/malware as well

I second using AVG--it's what I switched to once my built-in stuff expired after I bought this machine in 2006. I'm having a little trouble getting it to update on my PC, but that's more technical fail on my part than AVG since the laptop I inherited from my husband works fine with AVG.

I third AVG Free edition! Works beautifully!

I use AVG, too, however I had a recent virus which it didn't catch. So now I run AVG, cCleaner and Malwarebytes. You can't be too safe! 

Malware bytes is a great addition to AVG and Spybot.  Run scans from all three once a week (after updating and disconnecting from the internet).  You should be relatively safe, and it should catch things when you do pick something up.

Make sure you update Windows regularly.

Malware bytes is a great addition to AVG and Spybot.

It's been great for me so far! A big thumbs up!

I add my recommendation for AVG and in fact all the other programs that have already been mentioned -- I have them all and together they do a great job. What one doesn't catch, on that rare occasion, another will; everything mentioned above does their stuff extremely well and are simple to utilize.

Thanks everyone!!

I used to use AVG but I ditched it when it started to become a resource hog. I switched to the free Avira (it does have one annoying advert popup every time it updates), but both are really strong at antivirus. Malwarebytes and Spybot are also really helpful. I'd also recommend a firewall (if you're on XP and don't have one already). For that, both ZoneAlarm and Comodo are good shots.
I've used AdAware in the past, but I've found it redundant with the above, so I've binned it. CCleaner (that's Crap Cleaner) is great for clearing left over crap from uninstalls, temporary folders, etc. It's not really an anti-(something), but it's quite handy to have.
Of course, I do need to include the obligatory 'install Linux instead' comment.

AVG Free and Spybot Search & Destroy.