Do you connect to broadband via USB or Ethernet?
If it is the latter, I bet that all you have to do is remove TCP/IP protocol stack in Network Properties and re-install it.
Telewest screwed up my Win 98 SE PC when they tried to install my broadband a couple of years ago. Doing the above got it working again.
I can't give you a how-to, as I've upgraded said PC to run Win XP since.