Have Windows 98 no keycode on computer that had... (by 3 users)

I have an old Compaq Pentium processor that I installed Ubuntu linux on. I overwrote the hard drive. I have just reinstalled the original operating system Windows 95 from the F8 prompt in computer setup. I had Windows 98SE on the computer when I overwrote with linux but don't the cdrom. I do have a cd rom for Windows 98 but not the keycode which is lost. How do I get Windows 98 back up

Thanks for you help