Firefox 3 has an annoying feature which automatically scans a file for viruses upon completion.
Nifty and all, but its not really needed since real-time file scanning is enabled.
Plus, it seems to take forever and do nothing, so I disabled it.
Load up the "about:config" page and filter out "browser.download.manager.scanWhenDone".
Change this to "false" and whalla, no more auto scan!