When you installed XP (or upgraded to XP) the Windows XP "Upgrade Advisor" runs.  This is very critical to a successful upgrade and the advisor will flag any incompatible hardware.  Also, many hardware devices will need Windows XP specifc drivers as the older Win9x/Me drivers won't work. 

 

If this was a Dell supplied modem, check the downloads and see if you can find a Win XP dirver, if it is XP compatible.  If it was a Retail added modem, check the 3 Com/US Robotics site for a Windows XP driver.  If none are avialble you will have to replace the modem with a new XP compatible model.