You can buy an upgrade version of Windows XP (cheaper than buying a full version of XP and just as good - same end result.)
You would then put the Windows XP CD into your PC drive and it will either launch the install routine automatically, or you should double-click on setup.exe
The install routine will see that you already have Windows 98 installed and it will offer to replace this with Windows XP.
I would backup all your data before you start and make sure that you have the installation CDs for software which you use in case this needs to be reinstalled once Windows XP is running.
PR, June 2006