Agreed with WiredUp - Windows XP and Windows 98 will work perfectly well on the same machine as long as they are installed on separate partitions or hard drives.

When dual booting, always install the oldest Operating System first. Newer systems will be able to detect older installations and allow you to add them to the boot menu which will allow you to select which one you want to use at startup. Older versions may not be able to detect newer versions (like in the case of Windows 98 and XP) which is why you really want to install the oldest version(s) first.

It is possible to do things the other way round but it gets much more complicated as you have to set up dual booting yourself and create your own boot.ini amongst other things. In short, do it the way outlined above to save frustration