I warn my clients about this. Some pay attention, some don't. those that don't have no Windows CDs within a weeks or two. Hiding them in a drawer that isn't locked is the same as leaving them out on the counter. Office is just about as bad.
This leaves me in the position of reinstalling Windows from my own CD-ROM in many cases, but, if Windows will run I can get the customer's reg number with regedit, and if it won't, I boot on a floppy and search the registry with XTree to find it.