I used the "free" version of blocking/anti-virus programs for a while before I eventually bought the paid versions.
I use several programs to secure my system...because using only 1 or 2 never catches everything:
• avast! anti-virus
• Spybot Search & Destroy
• ZoneAlarm personal firewall
• MalwareBytes Anti-Malware
• Microsoft Security Essentials
Avast!, Spybot, and ZoneAlarm load up on boot-up. I run all of them once a week, a boot-time virus can every other week, and a root-kit scan every week.
So far, I've only been infected once...from grabbing a video file off a friends USB thumb-drive. The thumb-drive was infected and it infected my machine for about 2 minutes until Avast! saw its activity, shut it down, deleted the file, then notified me of the breach.
TQQdles™