I would disable auto-restart from safe mode
start>control panel>system> advanced system settings>startup & recovery>settings> uncheck "Automatically restart..."
let us know what error message you get
If you dont get an error message then I would next disconnect the front panel "pwr_sw" from the motherbd header. You can boot up the computer by momentarily shorting out the two pins the connector goes across (be careful not to short out any other pins) andif the restarts stop then its your power switch on your case. If they still happen then we need to test your power supply.