Why is my old computer so much faster on the internet?

I have an old dell 866 mhz computer in my patio room with wireless and a newer laptop with wireless and my main fast computer hooked up directly to internet. YET, my old Dell maneuvers faster, a lot faster than either of the newer ones with any program and especially on the internet. What gives. Both laptop and main computer are equipped with virus scans and fresh legal winxp installs yet are so much slower. Any ideas why????????