Hi
I would try to send a serial command, like "1" or "0", from a Rainmeter desktop Icon,"Set Led On" or "Off", to a Arduino pin led or intern pinled 13.
The Rainmeter that i use write in .ini, i don't no if it's possible to let the .ini data comunicate with the arduino .ino data
I just would to press a button on the computer desktop and let the arduino no that the button be'n pushed and set the lights on. Its all in the computer case and the Arduino is connected with Usb port4...
Please Help for my little project!!! 
I don't know exactly how you do it but I have thought of simply using a little programme in JBasic that sends a number to the serial port. It may even be possible to do it in BAT. The programme would be so small it could sit on the desktop, no shortcut needed, and you make up your icon for it.
If you use batch files, you will need to defeat the arduino auto reset when the serial port is opened. A resistor or capacitor can do the reset defeat.