Re: computer shuts down
Are you joking? A 108W PSU? That's nuts. Replace it with any PSU with the correct number of pins (20/24) as required by your board, and ensure you get any SATA power required, probably a P4 connector too.
Don't worry about making a direct swap, because that's a dreadfully pathetic PSU.
Also, judging by that Wattage, you've got a fairly old system. Now, either you've tried to upgrade the graphics, memory, whatever - in order to play this demo. Or, you're trying to run it on an old machine. Either are a bad idea, because you have an underrated PSU, poor cooling, and simply not enough 'oomph' to play such games.