Can the older games not be run in XP's compatibility mode for 98?
You can dual boot with 98 and XP, but normally 98 must be installed in the PC first and then XP. Windows 98 doesn't support multiple operating systems so, if you install it after XP, it won't recognise XP and you will only be able to boot 98.
Installing XP second will recognise an existing operating system and create the necessary boot menu to select 98 or XP.
You *may* be able to get around this with a third party application such as "Boot Magic" which is supplied with PowerQuest's "Partition Magic".