Is the garage door open? Reed relays
There are many ways you could achieve this. Which you choose depends on how many you plan to build, your budget, skill level etc.

For a wired solution, I'd pick a microcontroller and reed switch to monitor the door, then have this connected to the PC using RS422 or RS485. Having a microcontroller at the far end gives you plenty of flexibility to monitor other signals, or control other systems. Using RS422 gives you a potential range of over 1km.

To get things going quickly, I'd use an Arduino and a ready-made shield.
eg. http://www.robotshop.com/rs485-shield-arduino-3.html

At the PC end, I'd use an off-the-shelf USB to serial converter with the right voltage level conversion, eg. http://www.ftdichip.com/Products/Cables/USBRS485.htm