Not counting Solitaire or Minesweeper since they come with Windows by default.

I have directories for ZSNES, FCEU, and EPSXE I have copied between systems for years now.
Vampire The Masquerade Bloodlines
Unreal Tournament
and World of Warcraft since they started the free accounts last year. Every so often I'll play a bit and go back to other games.

New Vegas, Fallout 2, Thief 2, System Shock 2.

I play Morrowind and Planescape: Torment quite rarely, but even so they are the first things I install on a fresh PC after anti-virus software.