Oh I see, I use Microsoft Security Essentials which is free AV software, have no issues with any kind of slow down, as long as it isn't scanning.
I used to use the likes of AVG then Avast but find the Security Essentials much better so far.
Edit - LeJosh beat me to it!! Note to self, Must type faster!
haha Hells yeah I did.

i have that on my laptop, and we recommend it to customers, but i must have atleast 15-20 computers in the queue that are full of viruses and use this, aswell as avg, norton, mcafee etc. so it makes me warey of connecting my main pc to the net. we have 2 laptops at home so i really dont need the net on the pc, but if im going to miss out on a lot of SP features by not having the net then i might have to brave it.
MP isnt for me, i like to do things at my own pace and pause it when i want to for aslong as i want.
AV can only do so much though. If people are downloading willy nilly etc. one or two viruses will slip through. You have more than 1 AV on the same PC or just piloting other software? Norton and Mcafee are the devil, get rid of it.