www.avast.com
I've found this one very good and light, running well even on less powerful machines.
I've installed it in hundred of PC and still work nice. You just have to register to get a free one year subscription, and at the end of the year you have to register again to have another year. Simple.
AVG runs good too, i'd like more Avast, but on very old machines i had to install AVG, because Avast was slowing the system too much.

M.