Hp power supply problems

[Hpwned]

I have an HP Pavillion Slimline. When I plug it in the green power supply light comes on, but when I hit the power switch the fans turn on and immediately turn off and the supply light goes out. What am I doing wrong????

 

[C12Friedman]

Welcome to Tom's Hardware Forums,
What model HP? Have you changed anything in the computer? Have you updated or added any software lately?