It sounds simple but theres a lot involved.
Lets assume you PC is connected 24/7
A simple method is to set up a web server (Use EasyPHP).
This will allow you to view your site (home based web site that is).
Because you are using php script (EasyPHP) which is akin to C you can click on a button...
Try this one.
http://www.electronic-circuits-diagrams.com/alarmsimages/alarmsckt12.shtml
There are a lot of them out there... search google using "car alarm + circuit" etc.
Mike