Perhaps with AV, almost any, there can be conflicts when installing programs. I think its pretty common place to turn off AV during installations. Not sure why it would cause problems while actually running Poser5...it shouldn't. If it does, that, in my mind, would be a problem with Poser 5 coding, not Norton or any other virus program. In fact, if Poser 5 is having problems with any other program resident on the system, that would be a Poser 5 problem, not the other way around, especially when all was running fine prior to installing P5.

Humankind has not woven the web of life. We are but one thread within it.
Whatever we do to the web, we do to ourselves. All things are bound together.
All things connect......Chief Seattle, 1854