In order to indulge in the hobby of computer chess, it is necessary to download at least one chess engine (unless you author your own engine). The following sites will allow anyone to find the download link for any publicly available chess engine:
WBEC (links to Winboard engines also can be found here)
Computer Chess Wiki
Jim Ablett Winboard Chess Projects
Arena Chess GUI 3.0
Homepage links for each CCRL tested engine can be found at its results page.
Finally, you can ask for a copy of an engine at any computer chess forum (such as TalkChess, OpenChess, Winboard forum, etc ...) and you request may be fulfilled.