|Hopefully the 2nd partition is FAT32?|
Boot into Win98 then put your XP disk in the CD/DVD drive - wait for it to spin up. A Window will pop up asking you if you want to "upgrade (recommended)" - don't do it! Instead, click "advanced" & direct XP to install into the new partition. The rest will proceed as a normal installation. When the system reboots, you'll be greeted by a dual boot menu.