The process is called "Dual Booting" (i.e. booting your PC with an option of two operating systems).
As you have Win98 installed, you can easily install XP beside it. But as I say, you will need another partition - the best program for that is PartitionMagic from Powerquest Corporation, but there are other ways of doing it.
When you have created the second partition, boot your PC with the XP cd in the drive and follow the instructions for an advanced install. Then choose to install onto the new partition.
It is a simple enough process if you take care what you are doing; just back up all critical data before going ahead.