Make sure you got the right drivers first off. If you did and it will not install then your best bet is to find a different source for d/l or to contact the manufacturer for the correct drivers. Forcing the drivers to install may just cause more problems.
They may have not been WHQL certified by microsnot
but may still work
right clik my computer- properties- hardware- driver signing-tick the box that says to ignore and install anyway.
just make sure you are installing the correct drivers for your adsl modem
( some companies are too lazy to send drivers for checking to microsnot so they can certified to work with XP )