Something is shorting to ground directly sending live voltage through out the case there! The first thing is looking over all wiring to find which wire or contact is touching ground.
A standoff touching the board wouldn't necessary provide that much while stalling the system while direct 12v output from the supply sees a higher amperage direct from the caps in the supply itself. A reversed lead in a main power connector or broken wire or lead touching the case directly would be a few things to look for.
Friend thanxx for ur reply.But i do not understand anything about wiring.One thing i know that the socket in which my ups is connected only has two wires live nd neutral not the earth .Is that the reason for shock?
How strong is the shock - it may just be static? Definately get a three pronged power cable so that it earths the case. IF it is a big shock or you are not sure what you are doing take it to a professional - better to be safe than sorry.
I was thinking static. Try grounding yourself on something like a radiator before hand. If the shock is powerful and continues for more than a split second then you may be shorting something on your case, check the standoffs for your motherboard and that the power supply and all power carrying wires are away from the bare metal in the case.