As an former Windows user who never ran A/V because of the stated reasons, in my old age I've finally broken down started using one as rebuilding a box is no longer high on my priority list. If you are a home user on windows, I highly highly recommend http://windows.microsoft.com/en-US/windows/products/security...

It's free, it stays the hell out of the way, doesn't slow the system down, and works.

Edit: I do not work for Microsoft, and this post was written on a netbook running Ubuntu.