If the PC was originally W98, and upgraded to Me, and you choose to save the original configuration, then, it should be as simple as going to add/remove programs, and uninstalling Me.
If Me was the original OS, or the files have been deleted, then you are best to follow soulriders advice.
Another option, would be to temporarily install the boot drive as a slave in another PC.
Delete all of the Windows files in the Windows and System folders, and the files in the root.
The more you clearout, the better the new install will go.
This gets rid of the registry, and Windows Me.
You can leave things like favorites, desktop, and start menu in place.
I would also download the latest W98 drivers for the hardware, and create a folder to extract the zips into.
That way, when W98 searches for drivers, you can start with the new.
Then, place the drive back into the original PC, and install 98.
Even though you left the files on the drive, you will still need to re-install most aplications.
With no registry, Windows won't know how to run the program. Also, some programs load dll files into the Windows folders.
As always, back up all critical data before starting the task.
By the way, if you hate Me because of the user interface, forget about XP, you'll like it even less.