(as if we needed one) of the perils of downloading material from a total stranger's computer via Kazaa.
The Nimo codec pack itself is not a virus as you thought - it's a collection of codecs produced by many different people. Nimo themselves warn you that they can't be responsible for any bugs in the individual codecs. If you have a virus infection it was obviously included in the download, so please post back with details of the actual virus message you're getting from Norton, and I'm sure we'll be able to help.
Next time I suggest that you go out and pay for the videos - it may work out cheaper in the long run.