Windows 98.
OK first backup your important files.
If you don't have a start up floppy disk make one by:
Opening the control panel. Select Add/Remove programs and select Start up disk.
Or download it from:
http://www.bootdisk.com/OK you have your start up floppy.
Reboot the pc with the floppy.
Select the start with CD rom option.
At the A/ prompt type
format c: then press enter.
answer yes to warning prompts.
After the format is complete.
put the windows CD in the drive.
Now if your CD drive is normally named the D: drive type
e: then press enter. **
At the e/ prompt type
setup then press enter.
Follow the instructions and windows will reinstall onto a nice clean hard disk.
Remember to have your drivers disks handy and all the programs you have been using will need to be reinstalled.
** whatever letter your CD drive is normally you must type the next letter. That is why the D: drive becomes the E: while you are installing Windows.
Hope that helps.