The CD-key entered during installation is stored in the registry (and cannot be found on the install CD). One can extract the CD-key from an existing installation as it is saved as pure text in the registry, but some might find it easier to use KeyFinder.

In Windows 95 it is called Product Id:

[HKEY_LOCAL_MACHINE\ Software\ Microsoft\ Windows \CurrentVersion]
ProductID=

In Windows 98/Me it is called Product Key:

[HKEY_LOCAL_MACHINE\ Software\ Microsoft\ Windows \CurrentVersion]
ProductKey=

Note one can also use REGEDIT.EXE in DOS to extract the contents of the registry file C:\Windows\System.dat to a text registry file, and then use EDIT.EXE to search the text file for the above registry key.