Like bj mentions, no AV software will work perfectly 100% of the time... In addition to my above mentioned experiences with Norton, I have seen various issues with a variety of other products....

I had always used AVG, but when I switched to Vista, they did not support it yet. I then switched to Avast. Avast is pretty good, and I like its boot mode scanning option, but it can be flaky at times, I don't know if that is because of Vista or Avast (unable to delete a virus from the Recycle Bin for example)

When it first came out, MS Antispyware Beta 1 was AMAZING. My computer science teacher was giving a lecture and did a search on Google for some obscure piece of code, and ended up infecting the computer with a rather severe virus. This was a 0-day exploit and the four major name anti-virus products we tried could not get rid of the infection. The computers at the school had a feature that they automatically rebuilt off of a hidded drive at reboot, so we tried an experiment infecting three computers and installing MSAS on them. It prevented or removed the infection in each case. Now though, it doesn't seem to be nearly as effective.

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