All the Antivirus products available for PCs would easily pick up a rouge codec and eliminate it. (MacAfee Site Advisor (As well as other security programs) would also tell you if the site was dodgy)
As far as I am aware, virtually all the media programs available for Windows, will ask if you wish to download the codec first. (Even QuickTime)

Bill
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English Riviera:
Live entertainment, Real Ale, Great Scenery, Great Beaches, why would anyone want to live anywhere else (I�m definitely staying put).