First get your Win 98 Boot Disk and insert it into the floppy drive and then start your computer. Once you get to the A:/ prompt type in FDISK and then press enter. You will be asked do you wish to support large disks at this question just type Y and when the main menu comes up chose 5 and that will show you all your HDD/Partitions select the HDD/Partition with XP on it and type in that number.
You then chose Delete Non DOS Partition and remove the partition/HDD this will get rid of XP.
You then have to reboot the computer again and reset the wiped areas to a DOS partition you do this by hitting ESC key until you get back to the DOS prompt and then just hit CTRL/ALT/DELETE together this will restart the computer.
Once it has restartedtype in again FDISK and set the unused space as DOS Partitions in whatever size you wish and then reboot the computer again but this time when you get the DOS prompt type in FORMAT X: {where X is the drive/Partition that has just been created and isblank.}
Once this is finished you will have to remove the batch file that was created for the Dual Boot facility there are a number of ways of doing this but the easiest way would probably be to allow the computer to boot into WIN 98 and then in Explorer delete the batch file in question.
Once this is done you should be able to reboot without the need to chose which platform that you wish to enter.
The only thing is you have to be careful not to delete the DOS Partitions when you are inFDISK or you will lose the 98 section of your computer so be careful and double check what you are doing and then check again just to make sure, once you have deleted the partition information it is very diffictual to get it back and unless you havethe right software it will prove impossible to recover so be VERY CAREFUL when deleting Partitions.