Avast is as good as freeware anti-virus gets. Running multiple AV products will hurt performance and most likely they will not play nicely together. That said, a rootkit isn't a virus anyway, and many AV programs won't detect them.
I use:
Avast
Hijack This
Spybot Search & Destroy
"Noscript" firefox plugin.
And, most importantly, a very healthy does of paranoia. I don't steal software, music, movies, etc.