as there is an operating system on the drive you will have to install it as a dual boot set up. install it as a slave to your xp drive and enter your bios and set the slave win98 drive as the first boot device.
boot to the 98 drive and enter safe mode and try to install your motherboard drivers (as your new computer will use a different one to the old computer) it should then work ok hopefully. or it may not if your mobo is very new.
you will need to choose which drive you want to make the first boot device again if you want to get back to xp.
ps. if you have the cdroms of your older games you could try them with xp using compatabillity mode for win98 maybe, or have you already tried that?