I use these free versions:
Avast Antivirus
Superantispyware
Set it NOT to run at boot up. It will only slow getting to your desktop. The essentials are running, and you can scan anytime you want to do it. Important settings:
Once you have run a complete scan, you can speed it up radically by setting it to check only files that have been modified within the last 30 days. You can actually set it for shorter or longer times, as well. On my system, leaving it set for 30 days shortens a complete scan from over an hour to under 15 minutes.
Under complete scan, you can select one of two levels of a feature they call "Scan Boost" that uses progressively more of your CPU's resources to speed up scanning at the expense of speed on other programs. I just set it for the fastest possible speed and run it while I'm making dinner or taking a shower.
Comodo Firewall
Comodo's firewall is excellent. Do not install "Comodo Dragon" browser or "Geek Buddy" online assistance (or remove if you install them). Do not attempt to use their complete security suite with another AV program because multiple AV programs can conflict and cause problems.