It's certainly possible to do. If the system is using the NTFS file
system, right click the drive on which XP is installed in Explorer or
My Computer, select properties, on the general tab, it will tell what
type of file system, you will need to first delete the partition as
follows.
Assuming you have a Windows 98 boot disk from your previous 98 setup,
boot with the disk, type FDISK and press enter. Once in FDISK,
select Delete NON-DOS Partition and then delete it. Then select,
create new Primary Partition, set it active, reboot with the boot
disk, type format C: and press enter. When complete, with the 98 CD
in the drive, switch to that drive by typing the drive letter
followed by a colon and press enter, then type setup, press enter and
follow the screens.
If the file system is Fat32, simply boot with the boot disk and
follow the instructions above for formatting and setup.
Note, you will likely need to check with the manufacturer's of your
various hard ware devices including those included with the Sony
Computer for drivers that will work with Windows 98 and there's a
chance not all such devices will work under a Windows 9x system.