It is in the Win98_46.cab file on your windows 98 CD and is named
desktop.scf
It will appear as desktop in windows explorer.

You can restore it using the system file checker.
Click start | run and key sfc.exe and click OK.
On the dialog box that opens, click on the radio button to restore a single file and key desktop.scf and click start.
Point the source location at either the win98 folder of your windows 98 CD or to the folder on your hard drive that contains the cab files.
Restore it to any convenient location, such as the desktop.
Once it is restored, you can rename it if desired to Show Desktop.
Drag it to the quick launch toolbar and while still holding the mouse button, press and hold the shift key, and then release the mouse button and finally the shift key.
This will move it back into the quick launch bar.