I would have to say 'Microsoft Security Essentials' if you stay away from trouble, that is, you don't actively download malware ripe binaries, such as patches/keygens/etc. I tend to recommend this to most friends if they ask for AV help.

If you do access malware ripe content, then I recommend first uploading it through VirusTotal to get an overall sense of what you're dealing with. VirusTotal is a online service which passes your uploaded content to about 42-45~ virus scanners and binary analysis tools. This also includes the most common commercial virus scanners by the main brands, such as Avast, Kaspersky, BitDefender, AntiVir, AVG, NOD32, McAfee, Symantec, etc.

The folks behind VirusTotal also provide a friendly stand-alone tool called VirusTotal Uploader which will hook into Windows' Explorer's "Send-To" right click menu. Allowing you to select a binary from your machine and quickly get it scanned by the above tools.

If you do enjoy being naughty with software piracy, then the best "non-free" solution I feel is ESET's NOD32 (now branded as "ESET Smart Security"). It has a good rate of detection, it is resource minimal and has a nice minimal non-intrusive user interface.

If you want the best chance of safety, consider virtualisation, whereby you run another instance of your operating system in a protective sandbox. You then run your unknown or potentially unsafe binaries there to see their outcomes. I recommend VirtualBox or the better commercial product, VMWare Workstation.

In some cases, you may have a binary, cough keygen cough, that is generating information based on the signature of your current operating system, this will make virtualisation useless. In such cases, you can use Sandboxie on your normal OS setup to run binaries in a heavily privilege restricted state, with their modifications to your setup easily viewable and reversible.

I hope the above helps and provides some assistance to you and other fellow redditors.