The windows 98 serial number is located in the following registry key
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion
It is listed under the value name ProductKey

You can also download this windows script file prodctk.zip
Extract the file prodctky.js and double click on it and it displays a dialog box with your windows product registration key.

The windows script comes courtesy of a post by MrScary to alt.windows98