You will need a win 98 start disk, easy all you need is a clean floppy go to control panel>
add remove programs then click on the tab create a start disk.
when you have done that leave the disk in and reboot.
When it has gone through it's routine choose the start with cd rom support.
When the curser is flashing next to A:\ type formatC and press enter it will ask you if you are sure because you will loose all of your data type y and press enter.
When that has finished put your 98 disk in and type setup.exe and press enter, then it should start reinstalling.
Before you do any of this make sure you have the product key for windows and all of the driver that you will need to reinstall IE motherboard drivers, sound card (if not onboard) graphics (if not onboard etc)
I dont think that I have missed anything it's been a while since I have done it but I'm sure someone will correct me if I have.