Connecting to Net overwhelms PC

I don't know enough to even know how to explain, but will try.

When I plug in my network connection, my computer is fine for about a minute, but then seems to become so busy it is unusable. It hasn't frozen, I can still move the mouse, and wait extremely long for it to respond, but can't figure out why.

It does this whether I open any programs or not. I get low % CPU usage, but something is bogging it down.
I'm on XP and have service pack 3.

I don't know what other info would be helpful.
Any advice would be great, as I'm otherwise going to have to reformat the hard drive.