I have a problem with my Fujitsu siemens computer which isnt very old. 3 times today it has hung and then switched itself off. When rebooting it, the message says unable to boot..insert system disk. I have to press the on/off rocker switch at the back and then it seems to work then (after a few goes), but it's very intermittant.

Any ideas what the problem is?
I thought it could be a dodgey PSU. It's not my PC but a friends.