Windows 98 normally won't let you upgrade with an OEM or Retail copy - you'd need to use a special Upgrade version. There's a few ways we can get around this.

1. Navigate to C:\Windows\System on your existing 95/98 install.
2. Rename SETUPX.DLL to something else like SETUPX.OLD.
3. Start setup and proceed as normal.

1. You'll need to make a directory on your hard disk like C:\Win98 and copy the setup files there.
2. Open SETUPX.DLL from the installation files in a hex editor.
3. Go to the end of the file where you'll see something like 0.120…….File Copy Engine…
4. The 120 indicates the edition type. 120 is OEM. 102 is Upgrade. 101 is Retail. Change it to your desired edition.
5. Save the file and start setup from the copied directory.