Slow Internet could be your connection. Wireless is always slower than a LAN cable, how many users are sharing the connection, and the bandwidth of the connection (as well as the bandwidth of the server (sites) you're accessing) can also slow things down.
Mac's are pretty much bullet proof as far as malware is concerned... HOWEVER if you have a lot of "free" apps they could be phoning home your searches, or checking for updates. (Most users don't read the Ignorme file that comes with "free" apps. Often you agree that in order to pay no cost the "app" can collect information as a way for the developer to get paid.) All that information sharing can slow your machine.
A failing HD can slow a machine if your HD has never been replaced it may need replacement... so can one that's badly fragmented. (Lots of saving then deleting files). Doing a backup, reformatting the drive and reinstalling the OS, then restoring, will solve fragmentation slow downs.
The OS you're using could also cause problems if you're pushing the envelop on your platform. It came with 10.4, and you can run the last version of Mac OS X 10.6 "Snow Leopard". However, it cannot boot in 64-bit mode nor does it support "OpenCL". It cannot run versions of Mac OS X more recent than 10.6.8. (That's what I mean by pushing the envelope)
Firefox has a pop-up blocker add on, Safari has one native to the OS.
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