You've got to press the magic key while the machine is booting, and very early also. That is, before any message like "Testing 256 Mb memory ..." appears.

What the magic key is, depends upon the BIOS. Usual keys are del or (with Compaq) F10, but it can be other keys or combination of keys also. It's usual also (but not necessary) that at the bottom of the screen a message like "Press del to enter setup" appears.
Anyway, it should be mentioned in the documentation that came with the motherboard or the computer.

If you can't find it, tell the make of the machine or the BIOS (it might appear at the upper left corner of the screen as the first thing the BIOS tells you during the boot) and possibly someone will know.

It's not a question of the OS (the OS isn't loaded yet when you can enter the BIOS), but still I wonder why you choose the Windows 98 forum, and not the Windows XP forum.

Hope this helps,