You probably don't want to hear this, but get a broadband.
Seriously, modem connections are overlived already. Relatively cheap broadband connections are available in most areas.
You want to make sure it's not XP which taking the bandwidth and causing high pings? Then go through this tweak guide:
http://www.blackviper.com/WinXP/servicecfg.htmAlso, if you have a firewall installed, it might be just that. Make sure you have the newest version of all softwares which are running in the background, and you have your Windows up to date.
If you've had your system without a firewall and anti-virus while you have used the net, there's a possibility that high pings are caused by a virus or worm. Yes, even with a modem connection you need a good firewall and anti-virus softwares, and keep those and Windows up to date!
If I were you, I'd check my system with Stinger.
http://vil.nai.com/vil/stinger/