Whichever one you choose, understand that no single AV product is perfect.
Use VirusTotal to check any suspicious files against multiple AV engines. Handy uploader tool:
http://lifehacker.com/5427159/virustotal-uploader-20-instantly-scans-files-for-viruses-against-41-av-apps
Use Sandboxie to run suspicious files in a sandbox, preventing them from altering system files/registry/etc:
http://www.sandboxie.com/