[control a switch over the internet]

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 on you home page and write a function to send data to the com port.

Well its a start.

Mike

[can you help me]

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