Random freezes,restarts and BSODs.

I have those problems for 2 months but this week the problem is too big.My computer freezes , restarts or shutdowns at first 1 mins after i opened it.Freezes/restarting or BSOD at starting windows screen,at welcome screen and mostly 5-10 sec after displaying desktop.I scanned my pc with AVG and found nothing.I dont know what to do.I have to restart it 10-20 times(if i am lucky) to start...