Hi, A.
Yes, but...
It's easy to add WinXP and create a dual-boot system with an older version
of Windows, but you want to add an OLDER version to an existing WinXP, and
that's not so easy. In other words, if Win98 were already installed, you
could add WinXP quite easily; WinXP Setup would detect Win98 and create the
dual-boot menu automatically.
However, there are several roadblocks you will have to get around to add
Win98 to your computer. One is that your Win98 is an upgrade edition; Win98
is a DOWNgrade from WinXP, and will not qualify for upgrade pricing. You
must either have something like a Win95 CD to install the Win98 upgrade, or
buy a full non-upgrade Win98 package. Another is that, if your PC came with
WinXP pre-installed, it probably is formatted NTFS; Win98 can't boot from
NTFS, so you will need to reformat at least the System Partition (probably
C

.
For details of some of the problems involved, see MVP Doug Knox's web page:
How do I install Windows® 98/Me after I've installed XP?
http://www.dougknox.com/xp/tips/xp_repair_9x.htm
I'm not familiar with Dell model numbers, so there may be other obstacles,
but these two are big enough to halt your plan unless you are quite
determined. How much time and effort are you willing to invest?
RC