A trojan has appeared on my computer and periodically opens up a sys32 box on my screen. Every time this happens, Webroot warns me that there is an active threat named “TrojanDownloader” and a scan is needed to remove it. However, every time I scan immediately after the warning, nothing is ever found and Webroot tells me everything is fine. 

I know this isn’t true because the trojan continuously opens the black box every hour or so, yet Webroot tells me everything is fine.

How can I remove this trojan?