Specifying the Default Operating System for Startup
If you have more than one operating system on your computer, you can set the operating system that you want to use as the default one for when you start your computer:
Click Start, click Control Panel, and then double-click System.
On the Advanced tab, under Startup and Recovery, click Settings.
Under System startup, in the Default operating system list, click the operating system that you want to start when you turn on or restart your computer.
Select the Display list of operating systems for check box, and then type the number of seconds for which you want the list displayed before the default operating system starts automatically.
You can also directly edit boot.ini but only if you know what you are doing.
Using the above method you can set your PC to boot straight into XP but leave Win98 still there and inactive. This could be useful as if your XP become unusable for any reason, Win98 can still be booted and allow you to (for instance) research the problem on the internet.
One other tip. If you choose to remove Win98 entirely, do NOT remove the partition which contained it, as doing so will alter the 'partition number' reference in boot.ini and XP will no longer boot until you edit boot.ini to respecify the partition number.