What's the model of your Belkin? Do you have port forwarding set up? If not, that's something you could try. Have a look at the site below, if your router's listed then you'll find step by step instructions.
http://www.portforward.com/english/r...outerindex.htm
(if you do set up port forwarding, afterwards go to LW's tools > options > advanced > firewall config. Choose the 'manual port forward' option & enter the port number that you've forwarded > apply)

Do you see the firewall icon (brick wall)? Make sure that your XP firewall's still configured correctly.
Firewall setup

I've had a look through Platinum's terms & conditions & acceptable use policy... there's no mention of shaping or excessive bandwidth use. But, that's not to say that the ISP isn't up to something