I don't know if it can completely ruin a computer, but on general I know it to be performance hungry for quite a while.What the? Really? Crap, maybe that's why my last laptop took a crap because it's what I've been using even for this current one (McAfee Total Protection).
I have been using AVG for quite a while and I don't think it greatly affects performance.What would be a good antivirus and internet security these days?
Unfortunately other than this I don't know.