It shouldn't be necessary to downgrade from XP to 98SE to run games that are more capable with 98SE.
Right-click the shortcut icon for the game, then click Properties - Compatibility(tab). You can then set it for the version mode that you want to run the game in.
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Downgrading from XP to 98SE is more difficult and time-consuming than upgrading from 98SE to XP.
You're going to need a fully-bootable startup floppy disk that contains the FDISK.EXE and FORMAT.COM and generic CDROM driver files, and you're going to need a full version 98SE CD - unless you also have a 95 CD.
Finding and installing hardware drivers is going to be more difficult, and finding peripherals that work with 98SE is going to be more difficult.
Personally, I think you're giving up a lot of good use of that desktop by downgrading to 98SE.
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