If you have a "real" Windows XP CD, you can use it to repair the boot loader
after reinstalling Windows 98.  If the CD is a restore CD from your computer
manufacturer - it doesn't boot Windows XP but instead starts a restore
utility - you can't.  Our friends in the Windows XP forums will be able to
give you the exact sequence you need to follow to get into the Repair
Console and fix the boot loader, or a little work with Google will give you
the instructions.

The MBR (master boot record) does not know the location of the XP or 98.
Rather, in reference to this subject, it points to the first active, primary
partition on the first hard drive detected by the bios.  The MBR it uses is
on this 1st bios-found physical hard drive with an active primary partition
on it per the MBR of that physical hard drive.  The partition table of the
active, primary partition contains the location of the file system and the
location of the boot files.  These 2 things are required for the OS to boot
properly.  The location of the first active primary partition, and the
location of the boot files.
In an XP single and multi-boot configuration, boot.ini is part of the boot
process.  This allows the user to determine which OS to run.  This was
located on the C: partition, and your XP was on the D: partition which
boot.ini pointed to when you selected XP to run.  In your original OS
installations, that's what you had.
When you reinstalled 98, the installation rewrote the partition table's
reference location to the boot files, whch are only windows 98 due to the
reinstall.  The active partition did not change.
A recovery console will work with the XP CD if you know how to use that
facility.  There's a MS KB article on just that.
http://support.microsoft.com/default.aspx?scid=kb;en-us;307654

To avoid this unfortunate circumstance and hassle in the future, get a 3rd
party boot manager.  These work by hiding the partition from the other OS.
You can selectively boot from either.  The boot manager, if adequately
designed, will allow you to hide either partition (highly recommended) from
the other in the event of either OS boot failure.  Then you can repair,
reinstall, format and new install either OS, or remove the partition and
recreate one, format, and install either OS without further complications.
You may have to reinstall the boot manager afterward in the case of an OS
install as these write over the master boot record.  In the case of a 3rd
party boot manager, you want each parition hidden from the other during each
OS installation.  This forces the installation to put each separate OS boot
files on their separate partitions.  The OSes will be totally independent of
the each other.  You cannot make a mistake of overwriting the other OSes
boot files partition pointer with an OS install.

In your present setup, setting the d: partition active will only point the
OS to the D: partition table.  Since this partition table has no record of
the location of the boot files, you won't go any further.  In addition,
setting your present D: partition active will result it taking the drive
letter C:, and your former C: parittion will take the drive letter D:.  The
active, primary partition always takes the drive letter C:.  This the
beginning of the hierarchy of drive lettering in msdos.and windows.  XP is a
bit different within the OS.  Its hierarchy does not follow the same rules.

one way is to save MBR & restore it later (using an appropriate
program like MBRwork and the second is:

installing W98  by running
setup with the /ir switch  /will not change mbr ...