Did you UNinstall it first, rather than trying to upgrade?
Also when things like this happen, it tends to be that the trojan has put something on your system that can look for secure browsers / other security software and their installers running and just process-kills them before they can get anywhere. If you can't get it to work after a/ removing firefox, b/ restarting windows, then I suspect you're actually still infected, and unless someone out there can suggest a clever way around it I'd suggest you need to download and burn a "clean" startup/rescue CD (ON ANOTHER COMPUTER) so you can repair the f*ckage to your disk without any of the nasties having a chance to load in. Maybe even pop the disk out, put it in a USB reader and hook it up post-desktop-load to someone else's machine that's already got good cleaning software on and has autorun disabled.
Also when things like this happen, it tends to be that the trojan has put something on your system that can look for secure browsers / other security software and their installers running and just process-kills them before they can get anywhere. If you can't get it to work after a/ removing firefox, b/ restarting windows, then I suspect you're actually still infected, and unless someone out there can suggest a clever way around it I'd suggest you need to download and burn a "clean" startup/rescue CD (ON ANOTHER COMPUTER) so you can repair the f*ckage to your disk without any of the nasties having a chance to load in. Maybe even pop the disk out, put it in a USB reader and hook it up post-desktop-load to someone else's machine that's already got good cleaning software on and has autorun disabled.