The W98 ProductKey is generally found on the Windows user manual, the
certificate of authenticity, a sticker on the PC or a sticker on the sleeve
or jewel case the Windows CD came in.  The 25-digit key is 5 groups of 5
alpha/numeric characters.  The Key might be available from the system:

If the system is working in either safe or normal mode:

Open regedit.exe to this key and look in the right pane for the entry
ProductKey:

HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion

If the system is not working, start the computer with a floppy startup disk.

At the A: prompt, type:  C:\Windows\Command\Find /N "ProductKey"
C:\Windows\System.dat
(Note:  ProductKey must be typed in upper and lower case letters as shown
and there is a space after:  "Find"     after:  "/N"    and after:
"ProductKey"

If the hard drive has been formatted or replaced, US customers can contact
Microsoft at:  1 800 642-7676.  They will issue retail and OEM product keys
after verifying data located on the W98 CD.

I suggest writing the ProductKey on the CD with a Sharpie marker.

I know I'm a little late in this, but, click on Start, Run and enter
regedit and when it comes up look for the following entry:

HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion

then in the other pane look for the entry of:

Product Key

and you should find your key.

If you can't find that, then get to the DOS Prompt and enter:

find.exe/i "productkey" %winbootdir%\system.dat

after that press enter and you'll find Product Key and your key.