Avast! works just fine with Windows 7 as well, and is the best AV package on the market.
Note that the normal XViD codec will not work with Windows 7. You must get the x64 compiled binary.
Actually FFDShow does not actually work. The Windows 7 Beta cannot use third-party codecs. Microsoft is testing the Windows Media Foundation which decodes MPEG4 and H.264 streams, and it will intercept the decoding before any third-party codec. This lock will be released with the first release candidate.