I figured it out...
in the browser type "about:config"
Search for Scan
then change the value of "browser.download.manager.Scanwhendone" to false
If that fixed your problem good but you might what to check a little further because the file from Nvidia is not infected and Firefox should not have deleted it so you have something else going on. I just re downloaded it from Nvidia without turning off the Firefox virus scan plus I have MSE installed and had no problems at all.