My general advice is yes. If you like to run things you randomly encounter
and elevate things willynilly, yes.
Personally I haven't used any since I've been running Windows Vista. I
might use Codename Morro when it gets released - if it stays out of my way
like Windows Defender, otherwise I probably won't bother.
I've never seen a virus on Vista. A few junkware things like My Web Search
and P2P toolbars etc., but never a true Virus or Trojan. Make sure your
automatic updates are installed as soon as they are offered and don't
download the usual crap you see like free porn (just needs a 'decoder' -
yeah right) or download accellerators or registry tweaks etc.
Vista works pretty much perfectly as it is, so don't mess with it.