normally one would install win98 first
XP second

if you try to do it the other way
you should look for a third part boot manager

It can be done without additional software *if* drive C: is a fat 16 or
fat 32 partition.  XP is probably going to be installed on a NTFS
partition and if this is the case, a third party boot manager and
partitioning program will be required.

Go to MVP Doug Knox' web site:  http://www.dougknox.com and click on:
Win XP tips in the left pane.  In the right pane, click on Install 98/ME
after XP....

Even if XP is on a fat 32 partition, I would still recommend using a
boot manager and a primary partition for each operating system--it is a
cleaner solution.  I use:

BootIt Next Generation is available from:
http://www.terabyteunlimited.com/  and it does partitioning, makes a
compressed image, does many other partitioning chores and is a boot
manager.  It is not quite as easy to use as Partition Magic but it is
less than half the cost and has more features.  Unlike the crippled
PMagic demo, BING is a *full function* demo you can try for free for 30
days.  The web site has a lot of support articles.