Okay. You will need the make and model number of your modem.
While on Windows 7,
Find the driver
on the net and download it to an easy spot to find. XP-32 bit driver. If your XP is 32bit. Say the desktop.
Hopefully you have a Flash drive?
If you do copy file/download to the flash drive.
Reboot into XP. Copy driver to XP desktop from flash drive. Then install driver.
Then you will have to set up to dial and or connect to your ISP as you did in XP before or Windows 7 when you installed it.
Once you get on line you can then get the rest of the drivers
and get Microsoft updates for XP.