Trojan? or False Positive?

When you say you deleted it, does ZA not keep a backup so you can restore if it was a problem ? AdAware that has been mentioned and a number of other anti-spyware programs allow for quarantine or backup of files/registry keys if you need to restore them.

Your computer is as clean now as it is likely to be since you have performed many scans with different programs.

The fact that nothing came up in a google search I would say in certain cases (depending on what the file name and location where), nothing being returned is suspicious and would warrant further investigation (Virustotal/Jotti, etc.), difficult when the file has been deleted.