SLLLOOWWWNNEESSSS of broadband connection

I have a cable connection and lately, my speed has dropped considerably. I am by no means have the technical background to figure out where to start troubleshooting. Can anyone point me in the direction of where to start. Cable compnay, computer manufacturer, router , modem, cable etc.?????? Could there be a virus on it? Any direction would be appreciated.
By the way I am running XP home on a P4 2.66Ghz. I have a INtel 850 chipset w onboard NIC.

First thing first. Do a speedcheck using www.dslreports.com

Run a recent antivirus program. You might have something like Blaster. You can use the Stinger utility from Network Associates to scan for the worms.

I get this about twice a year with NTL. I have to phone and get them to sort it out. Usually its because over "over subscribed lines, sorry we can't do anything" and I say, "but I pay for a 1MB line, and I now have dial-up speed". "Oh, ok, we'll have to dig up the road, but we'll fix it". Yeah... i'm grateful.

"over subscribed lines, sorry we can't do anything"

My repsonse would be, "Why do you over subscribe lines in the first place? Surely you could do something about that. no?"