Sounds like its either the power or reset switch,
Turn the avr off
open the side cover and locate the two connectors for this at the bottom front of the motherbd and disconnect them.
Turn the AVR on.
Some pc's are set to turn on when power is restored, I suspect that yours is and will turn itself on now.
If it works now you can shutdown using the start button>shutdown. and when its off turn off the avr
Reconnect the power sw
Turn on the avr.
if the problem didn't return then its probably the reset switch. You can repeat the above test to verify or just leave it disconnected, not many people use it.