For the price of a new motherboard and the hassle of trying to do it, you can probably pick up a budget PC that will be much faster than your current machine and will hopefully run XP which is a far more stable and generally superior operating system.
I think you can get a new machine at the low end of the spectrum for the matter of a few hundred bucks - a new motherboard and Windows CD (if you've lost your 98 one) will come to almost as much and you'd only be flogging a dead horse.
Sorry I can't bring any more cheer than that
