Ok first of all, if you have a corrupt Win98 already, you may just be adding fuel to the fire by doing a "Dirty Reinstall". But I will give you step by step instructions on how to do this.
Ok first you will need the Win98 cdrom.
Now, in Win98, assuming you can get into the desktop, goto
A. Start > Find > Files or Folders > (filename) win.ini entered into Named.
B. Click Find Now.
C. Right click on win.ini, and select Rename, and rename it to winini.old
D. Repeat those same steps for "win.com" renaming it to wincom.old
Put the Win98 cdrom in the drive.
Reboot the system, enter your systems setup, and change the boot device to your cdrom, making your cdrom drive the first boot device.
Save the setup settings, and when your computer reboots, it will autoboot to the Win98 cdrom, once in, you will want to select "Reinstall Win98 over current operating system, and then go with the recommended settings throughout the rest of the install, you may need to reinstall your drivers
such as "modems, video, eth0, and any other devices you may have, you may also have to reinstall some of your programs, but that is how you do the dirty reinstall.