I have been a long time user of AVG anti virus. If you know where to look you can get the serial key to upgrade from free to the pro version.

It was good, but did eat up a lot of memory and hogged the CPU, so whilst it worked fine on my new i3 laptop, my old 8 year old PC was very very slow.
I have recently installed Avast onto said PC and it uses a lot less memory and CPU time, so I can recommend Avast for your old laptop.