If you can help with the following paid ZA alert I would be most grateful - Trojan-Downloader.JS.Agent.ftw

Today when logged into my eBay account and with the watched items page open a ZA alert opened for this trojan, located in the temporary internet files - unable to quarantine.

I did an advanced option scan which resulted in a new alert, and great relief when ZA found, and quarantined, the same trojan in the TIFs

What does this trojan actually do - how quick does it activate and what is down loaded?

Why no quarantine on the 1st alert?

Was it embedded in an ebay ad/page etc or had it been dormant on my pc?

No info via Google etc. - so any input welcome. Thanks.