Is this correct: (1) Windows 98 was first installed on the C partition, XP was later installed on another partition, and the dual boot was automatically created by XP installation, and (2) you did a repair installation of 98 (i.e. without formatting C; and (3) now cmputer boots straight into 98 only, without the OS menu?
If all the above assumptions are correct, read on. If anything is wrong, post back.
A repair installation of 98 on the C partition has changed the C boot sector to a 98 boot sector, so that on booting, the computer will only look for the 98 boot files. You only need to repair the C boot sector to change it back to an XP boot sector.
First check in My Computer, under your C drive, you still have the following files, ntldr, ntdetect.com, boot.ini and bootsect.dos (make sure that "Show All Files" is selected in View, Options.)
If you have all those files, insert XP CD, restart, and boot from the CD (making sure that in BIOS, CDROM comes before the hard drive in boot sequence).
When the XP Setup screen appears, enter Recovery Console by pressing R. It should then list the XP installations. As there is only one, it should say 1. X:\WINDOWS (where X is the drive letter for your XP partition). Press 1, Enter, and then enter your Administrator password.
At the X:\WINDOWS> prompt, type
and press Enter. When asked, confirm the action.
At the prompt again, type
and press enter. Computer will restart. Remove CD. See if you get back the OS selection menu.