I can't get rid of trojan virus that virus scans aren't detecting - help?

I have some kind of virus that I must have gotten when I tried to download some video. My computer is working fine except for this pop-up alert I have been getting for about 3 days now that says:

''System Error! Your computer was infected by unknown trojan. It's dangerous for your system (critical files can be lost)! Click OK to download the antispyware program to clean your system! (Recommended)''

I've since run several virus scans including McAfee, Norton, AdAware, Windows Defender, Trend Micro Housecall. Some adware and spyware was detected and I got rid of it, but the message keeps popping up no matter what I do.

If I click OK it automatically prompts me to download this supposed ''antispyware program'' but I am afraid it's not legit. Or is it? It just seems really sketchy.

Any suggestions on how to get rid of the trojan?

I can't get rid of trojan virus that virus scans aren't detecting - help?computer security

download the free version of Superantispyware. http://www.superantispyware.com Update and run an in depth scan. Recommend you reboot if any nasty virus or spyware is found to be on the safe side. No conflict with your own antivirus software.

Free means you must update on a manual basis daily.

Mind Doctor, France

I can't get rid of trojan virus that virus scans aren't detecting - help?free spyware remobal