I used Kaspersky once and it slowed my already snail like machine to far less than snails pace. But the main problem was getting rid of it when I had had enough of watching the paint dry. It would NOT go away! It was soooo like AOL you might have thought they were invented by the same person! Tenticles everywhere. We eventually (IT son and son-in-law) decided a clean install was the only way, so I had the onerous job of trying to get all my documents backed up - it took DAYS!
I use AVG free. No problems so far, that's 4 years now.