1. You can still buy Win98SE discs on EBay.
2. You can borrow the proper CD from a friend and make a copy for future
needs.
3. You can try using a car wax to fill and polish the underside of the CD
you
have now. Sometimes it fixes things.
4. You should use Find>Files & Folders to search for WIN98*.CAB files. If
you find a bunch of them (dozens) in one folder, that folder is where you
should tell the DUN installer to look for files.
5. If you don't have 4., then as soon as you find a disc, make a new folder
on the hard drive called WIN98, then copy all the files (excluding
sub-folders) from the Win98 folder on the CD to the new WIN98 folder on the
HD.