LEDs and how to determine how to to prevent them from blowing out

Hello again...

So I was helping my son rework the cooling system on his RROD Xbox 360. We added some additional fans to a 5v constant power source and a different heat sink to the GPU. we wired a switch to the fans and an LED so we would know it was running.

Now the fun stuff.. We have blown 3 LEDs. All of them seem to last for a little while, 2 or 3 days. but then they blow.

How can I determine what LED I can use and is there something else I should put between the hot lead and the LED to keep it from blowing?..

The LEDs I was trying to use were pulled from old equipment I had laying around.

Thx,
Kevin

Yes, you need a resistor. A quarter-watt size is fine. For a red led on a 5V supply use 180Ω. For white or blue, use 100Ω, for green or yellow use 120Ω.

This will limit the current to about 20ma.