Houdini

And if your father is an übergeek, the strongest chess engine in the world is absolutely free!  Download here.

You will need a UCI-compatible chess graphical user interface.  ("A thingie that allows you to look at the game.")  I use ChessBase, but both Arena and Winboard are free.

From the Houdini homepage:

Houdini is appreciated by many players including top Grand Masters and the current chess World Champion. After his match against Alexei Shirov, chess World Champion Vishy Anand was asked "OK, three: Fritz, Rybka or Houdini?". Anand's reply: "Houdini" (read the full interview).   In another recent interview, when asked the question "If you had to pick one player to represent earth in a chess game vs. aliens, which active player would it be? You can pick different active players for the white and black side, if you feel it necessary", super GM Peter Svidler had the same simple reply: "Houdini".