when I start up my computer & doing my work, It will automatically shut off without notify me. I turn on my computer again & windows does not scan my hard disk for improper shutdown. I check my power schemes setting, all set to never/always on.
I have change the computer power supply but the problem still occur.

Welcome to the PC Guide forums!

Time to start looking for a short somewhere! Have you or anyone else installed internal hardware recently (other than the power supply)? Like a motherboard, perhaps, or something that would push against the motherboard?

It could be getting too Hot. Have you checked your shutdown settings in the BIOS?
Stay Cool:cool:

Olie :D