|I don't think Ad-Aware has been very highly rated around here as an AV, although it has obviously changed a lot since I last used it years ago.|
Probably the three most popular free AV's are:
Microsoft Security Essentials
I regard AVG as a bit bloated and the other two as "broadly" similar.
When you have installed and run the virus checker of your choice, as a first step I would then download, install, update and Run "MalwareBytes" free version. This is good at finding and fixing awkward malware that AV's sometimes miss although it is not an AV as such.
You need to keep your AV running always, but MalwareBytes need only be run when you think your AV might have missed something and/or you have a problem. Keep it updated.
How to know you are getting old 5:
A lamp post backs into you