I have been finding site that say they have serial numbers, cracks, full version, etc.. of WinFF, and every one of them especially on P2P networks are viruses.
There is no such thing. There is no serial required or nags screens to be removed in WinFF. WinFF is Free and Open Source Software. A copy of winff can't be stolen, because , it's free.
Clean virus free versions can be obtained from Google Code and WinFF.org. If you don't download from these, at least use a reputable source.