How can I use my old Macbook as a second monitor?
Excellent suggestions from Jan. Basically, you can use the Macbook as a terminal or remote access device, but not as a simple screen. Using Synergy or similar products comes with a significant performance hit - particularly on graphics, since the "remote" computer has to send a description of the display to the Mac over the network. It's usually OK for wordprocessing etc, but not great for videos or games.
MacBooks don't have video-in ports so this is only possible via software requiring the MacBook to be in working condition.
If you were to use the MacBook as your primary device or your other system is a Mac too there is a software called ScreenRecycler.
In any other case I would rather suggest a setup e.g. using Synergy which let's you control multiple working systems almost like they were one.