The Trojan is Trojan.Win32.StartPage.apm
It's also appeared on a third machine (different location) so seems to be repeatable.
Note that, if you have downloaded DropBox and then deleted it (i.e. without installing), the Trojan is still there - in your Recycle Bin. So you also need to run something like CCleaner to over-write it umpteen times to squash it. HTH.