I use Avast on my pc and the misses laptop, its free and it doesnt slow the pc down, never had a problem with it and never had a virus etc.
And i know it doesnt slow anything down because her laptop is not a high spec one yet it doesnt slow it down still.

In the 4-5 years ive been using pcs online ive always used a free AV and never had a problem with virus or any other nasties