Everything Apple (the Mac heads thread)

Matt, put your install disk in the drive (drive should open/close as normal when powered on), then turn the Mac off and on again, hold down the c key as it's booting/turning on until a welcome screen appears* (you've just booted from the CD rather than your HD), then run hardware test from the utilities menu.

• Alternatively hold down the option/alt key and then "select a volume to boot from" (a CD will appear next to your HD, click that).
  
  

With a bit of luck you'll get a diagnostic before the Mac dies again, maybe with a lot of luck.