If this is for your home computer you really don't need any anti virus. You don't just get a virus from being on the internet. You have to download sketchy files and run them, open shady email attachments, go to shady websites with outdated browsers full of security holes, never update your os, etc...
With that said AVG and kaspersky are good to check out. There is always norton and mcafee too. They work but are bloated and may slow down your system more. Any anti virus will slow down your system some, but if the system is pretty high end you won't really notice it.
If you ever get a bad virus I wouldn't bother trying to scan for it under the same infected operating system. Instead I'd use something like kasperky rescue disk. You boot up to a linux distro on a flash drive and then scan the fked up drive that way. Trying to boot up a badly infected windows install and run a virus scan from that same install will make you want to shoot yourself.