Pc shutsdown when I log-in

Just wondering what's the real cause of this.

When I start my pc sometimes only the motherboard logo flash-in (samsung) then after that bootscreen then after that when I already log-in after several minutes it shuts down automatically. I then start the pc over, but still the same thing happens...it still shuts down automatically. There are times that the Bios flash-in and it ask to hit F1 or F2.

The power supply is new, antivirus is updated, connections are fix, all fans are running, etc...

Sometimes it says that CMOS setting is wrong, I have already tried to reset it to default but still the same thing happens.

Any advice greatly appreciated.