One on my work pc's has started shutting down suddenly about a minute or 2 after windoze starts up, everything works perfectly until it swithes off. Whipped the side cover off and powered it up, notice the psu fan isn't running. Should this be going all the time or does it only come on when it's getting hot? If it should be running I guess the psu's knackered?
This forum has never let me down in the past so there's a comp wiz logged in!

most older computers run the PSU fan all the time, but newer ones can stop the fan when it's not needed. You may find a setting in the BIOS that allows you to switch off the power saving to allow the fan to run all the time.

Does the fan run on startup?

Yeah the fan really should be running, it's shutting down to prevent fire

New fan should be a couple of quid, provided that's the problem..

All the fans should run from start up, if it is the fan in the power supply fit new power supply (less than £20) if is the one on the processor check the connector plg to the motherboard isn't knocked off if connected but not working fit a complete new heat sink.

[Edited on 24/3/12 by britishtrident]

The psu fan doesn't come on all on booting up... new psu then we think?

You need to ensure that the CPU heatsink is clipped hard onto the CPU.
I had a socket 478 Pentium4 where the heatsink mounting bracket had broken off. Because of this, the CPU was getting hotter than it should, which caused the computer to shut down.

If you are re-seating the heatcink onto the CPU, you will need the appropriate heat sink compound, which gets smeared onto the back of the CPU, where the heatsink presses against.

Check this out before spending money on any other parts.