It's propably a stupid question but how can detect volt change with the arduino. My plan was to hook the arduino up to my pc led so that i can detect if the led is on or off.
can someone plz help this newbie out :-[
A very cheap/simple way would be to use a light-dependent resistor (LDR) as part of a voltage divider, with the centre point connected to an analogue pin.
okay thanks i will try that out 
the simple way would be to use a transistor to pull a pin down tru a resistor when the light is on
so if I connect pin 2 with a 100k resistor to the 5v of the led and a 10k resistor to the led gnd than i should be okay?
the pull down resistor did the trick ;D