Well There's Quake 3 (online browser version) that works pretty well in the way of fast game, no story and instant murder
If your looking for offline games You could go for Max Payne 1/2. The games themselves aren't really long, quicksaves work at any time and its just in general fun if you don't follow the story line.
There's "Angry Birds" if your bored enough
. MinceCraft can work. Project will develop slowly but if its for 15-20 minutes Im sure you can get something done slowly. Good for taking up multiple segments of time.
If you can get em to work: Quake 1/2. Doom 1/2. DukeNukem3D, Blood(same maker as Duke nukem) These are ancient but still work on XP. even vista, (never tried 7) If you want a copy I can send you quake 1 with both expansion packs that work. Each level takes between 2-10 minutes depending on you
.
If your looking for horror, there is the Penumbra Trilogy, Very good short survival thrillers with no fighting. Very little storyline and makes you think alot.
---More may come on this post if I remember anything else hehe
--Is totally not a nerd or anything 8D--