C-s must be pressed twice

Strange… maybe trying to rebuild your services menu (and then restarting) will help? See the top answer here:

https://superuser.com/questions/43700/snow-leopard-services-menu-says-its-building

Try the following:

1. Open the "Keyboard" preferences pane
2. In the "Keyboard" tab, turn on the "Show Keyboard and Character Viewers in menu bar" checkbox
3. Go to the new menu (it looks like a box with a command key clover-leaf in it) and choose "Show Keyboard Viewer"

You'll now see a window shaped like your keyboard, with every key shown, and each pressed key highlighted. Try out all the keys, especially the "t" and modifiers. Note that holding down "shift" will change the key display to capitals, and holding down "option" will change the key display to alternate characters. This should let you track down the problem, if it's in hardware.