To install 98 over itself, boot the PC and keep tapping the delete key to enter set up, set the boot order to CD ROM drive then hard drive, press F10 to save, type Y to confirm, put the 98 CD in the CD ROM drive, press enter, choose option 2 to load from CD, choose set up windows, ignore the warning about windows already being on there, continue with set up, you will need the product key
It is important to do it this way round so you get the chance to back up the data