Why do you want to upgrade? Chances are that your old PC will not have the oomph to run XP very well.
If you want a faster, stabler OS than 98SE on an old PC then Linux is really your only choice.
$9.95 will get you a copy of the excellent MEPIS Linux -
http://www.mepis.org/ - and more applications and software than you can shake a leg at.
Or you can go for one of the bigger distros - $29.95 will get you SuSE Linux 9.1 Personal -
http://www.digitalriver.com/dr/v2/ec...P=0&CACHE_ID=0
Come on in - the water's fine!