Siemens ISDN SANTIS in Windows XP

I was using an ISDN external Siemense modem in Win 98 for a long time. recently, my company has upgraded the PC’s from Win 98 to XP. But before doing that I haven’t uninstalled the driver of the modem from the PC. The problem now is that I can not uninstall the driver, neither can I re-install it in XP !! and the PC doesn’t recognise the modem when I connect it to USB port, as a new hardware. The windows help is telling me that there are some corrupted or incomplete lines in registery, but it cannot help more how to correct this problem. So I am blocked without my modem !! Any suggestion how to solve the case ?