Q:

Connect a switch to my PC for a buzzer type of app

I want to write an app that assesses your reflexes.  Basically it makes a sound you have to punch a target and the programme tells you your reaction time.  I am not sure what to use for the target part.  I was thinking of either a cheap mic as the sound of the impact would loud enough to be picked by the programme and that would the cue to stop the stop watch, or maybe an optical mouse same principle if you punch in front of the mouse the cursor will move and that is the signal for stopping the stop watch.  
Any ideas would be welcome, the system doesn't have to be perfect and can be a tiny bit makeshift.  
Thanks in advance

A:

I would get a Raspberry Pi because it has GPIO pins. 
Then connect a Limit switch to a GPIO pin. 
When the person punches the target the limit switch will close and you can mark that down. 
The limit switch, of course, has to be placed behind the target.