What OS are you using?
I only use Eset Internet Security.
Plus non-resident malware scanners - Halwarebytes and Superantispyware.
Firefox has adblock-plus and WOT.
My old P4 (2.4GHz) with 1GB RAM flies along very smoothly. So either you have a different problem or you've got so much security software running there's little room for anything else.