I have been using Avast for over a year now, free for home users and free updates. I notice no major resource hogging and the scanning of email is quick and painless.
As for catching stuff, I had run Nortin and threw it out since it wanted to control my computer, installed Avast removed Norton and did a complete scan. Avast found 7 virus/trojan files hidden in the PC that Norton obviously overlooked it was then that I swore to never go to something that wants you to keep paying for it

I have also tried the other freeware virus programs but Avast has been the best I have found and I plan on using it for as long as it is around or something better comes out.
Only thing that some dislike (why I don't know?) is you must register with Avast and this is done once a year upon doing so you get a years worth of program and virus database upgrades free, when a year is up you will be notified to register again then it is another year of virus free computing!