I use only freeware apps to protect my pc.
Over the years I reckon I have nearly tried them all.
The one thing I gladly have paid for is backup software, it has proven to be invaluable. No matter what software I throw at my pc, harmful and otherwise I have successfully been able to restore my pc back to perfect working condition with it. The try and decide feature is awesome also.
The backup software I use is Acronis True Image.
Antivirus.
I protect all my pc's with Avast Antivirus. This is defenitely a user choice and others will swear theirs is better. I'm not going to say Avast is better than the others, it just became my AV of choice.
I like the boot-time scan Avast offers. Resident protection is excellent, although some find the detection warning screen that pops up can be intrusive and can cause undue worry. I set my screen saver so that Avast scans whenever the pc isn't being used and the user interface is my favourite. I also like the fact that it updates silently and you only notice that the updates have taken place when a little box pops up on the bottom right of the screen letting you know.
I have tried AVG and Avira also. I couldn't really take to AVG, even though it is the most popular free AV program. I found the update function intrusive, placing a box in the middle of the screen until the update has completed. This probably can be switched off, but I didn't use the program long enough to find out. Avira has the best detection rate, but I found the hueristic scanning a little too sensitive giving quite a few false positives. Also it constantly reminds you it's there, there's a little nag screen that pops up which I found quite annoying.
Firewall.
I use ZoneAlarm free for my firewall. I run the odd leaktest online to see if it is still up to scratch and the reports always come back that all the ports are stealthed, so this is the best outcome I could ask for in a firewall. ZoneAlarm has lost a bit of popularity to Comodo. I ran Comodo for a week or so but reverted back to ZoneAlarm. Comodo is more configurable and has a few more extras, but I found ZoneAlarm to do everything I need in a firewall.
Spyware / Antimalware applications.
I used to be a big fan of Adaware and Spybot Search and Destroy, and they served me well.
These days I use only Malwarebytes' Anti-Malware and SuperAntiSpyware free. They are very powerful at malware detection and removal. These are on call scanners which means they don't run resident. SuperAntiSpyware does load at startup to protect your home page but I switch this off.
Windows Monitoring Utility.
I use WinPatrol to monitor my system. There are so many options and it does so much so I will only touch on a couple of features. WinPatrol monitors your system and alerts you if there are changes being made. The sacrifice is that when you legitimately want to install something you will be alerted and have to let WinPatrol allow it. You can control your startup programs efficiently also.
Basically it allows you to take significant control against malicious programs.
That's the run down of what I use to protect my pc's. I'm by no means touting them as the best free programs out there. They are just what I have ended up with. It still very much comes down to personal choice.