Spaced Invader is right--Even if you do remove the dual boot, there will still be stray files. If you wanna remove anyways, here goes-- Boot into Win 98, and open up 'boot.ini', which should be located in your "windows", or "windows/system32" folder--can't remember for 98 (may be a hidden file). Once it is open (in notepad) you should see something like this:
[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(2)\WINDOWS="WinXP" /fastdetect
multi(0)disk(0)rdisk(1)partition(1)\WINDOWS="Win98" /fastdetect
I hope that shows up fine up there--you may have to maximize the screen. In any case, delete the line for Windows XP (multi(0)disk(0)rdisk(0)partition(2)\WINDOWS="WinXP" /fastdetect). This will stop the dual boot from happening. If you don't want to remove this, you could switch the lines, (Win98 to top, WinXp to bottom) and then set the timeout to 2 seconds. Either way, this should solve the boot problem. Give it a try and let me know how it goes.