If you are running the setup for Windows 95/98, and are asked for the CD key but don't have the cover with you, open Regedit and go to :
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion.

There you will find the string value ProductId; ignore the first and last set of digits, and the remaining numbers are the CD key. The same goes for Plus!, which has its CD key at Plus! ProductId.

Of course, there is an easier way to view this info, but if you need to edit it (if reinstalling from another CD), that's the way to go about it. To quickly view the CD keys, open System in the Control Panel; in the General tab, look under "Registered to:" and the number will be the Win 95 CD key. If there is a second one, it will be for Plus! Same rules for which digits to ignore apply.

Of course, if you have had a PC catastrophe and need to reinstall Windows, and you have lost the CD cover, this info will be next to useless. So, for your benefit, here is a list of CD keys. They will work no matter how different the numbers are from those on your original covers.

Windows 95/98 : 415-0920881

MS Plus! : 415-0962666

MS Office : 3134-0414694