gingernut,
I just want to verify that your computer's OS in Win98.
If it is, then the disk the computer is asking for is you original Win98 CD that was used to install the OS.
You could try the browsing option so that the OS looks in the directory:
C:\Windows\Options\Cabs
That is where most people put all the CAB Files from the Win98 CD.
Take Care,
Richard