Wireless Door/Window Sensors

Hey all,
Wasn't sure if this was the right place to post this, but technically it falls under wireless security.

I would like to know if there is anyway to install some sort of door sensor that will report back to my computer if the connection is broken (door is opened). I don't want to go out and buy an entire security system that does all of this, but I just wanted a very simple project to understand sensors and how to tie them into the computer. Ultimately I'd like for it to detect when the door was open, attach the date/time and log it on my computer.

I can't imagine this being terribly difficult, but I can't really think of a good way to implement this, every sensor that I have researched either just does an alarm when the magnetic field is broken or it reports back to some central proprietary system.

Any ideas?

Check here

http://www.davis.com/

I have bought some simple PLC devices from them. e.g. I monitor my well pump.
Shows on/off and timestamp.