When the power is good there's often a little Green Light (LED) on the motherboard saying so - when it's plugged in but before you ask it to power up you should be able to see the light if you take the side panel off. A new or second hand PSU will be from £20 - £50 and is an easy ten minute job to replace.
If it is the PSU then you may be ablke to repair it yourself - if it isn't you may need to take it to a computer doctor.
Good luck...
Peccavi, January 2010
