are they all on the same partition? hmmm...
ok then, if they are you can't use NTSF file format. win98 shouldn't even work if this is the case, but what it may be is that since installing WinXP, a new boot sequence was added to the MBR (Master Boot Record) replacing the previous boot records. this shouldn't be a problem since they are all Micro$oft products, but, win2k and winxp may be both trying to control the MBR and leaving win98 stuck in the middle to try and figure out which boot record it should follow.
suggestions:
1. format :/ and create a partion for each OS
2. format but install WinXP first, then Win2k, then Win98 making sure that Fat32 is used and not NTSF
3. could also be a mobo driver problem.
good luck