My home PC is automatically powered on whenever i plug in the power cable. I need not press the CPU power on to on my computer. How can avoid this behaviour of the PC ?

I suspect a faulty power supply. Get it checked out at your local computer shop and get them to replace it if it's faulty...

I changed the power supply. But no betterment in performance. Any other idea?

I changed the power supply. But no betterment in performance. Any other idea?

try this:

control panel > power options and change the setting from all ways on to home office computer!