The other day a burning smell came from the external modem (usb with no external power supply). Powered down, and on next power up seemed ok. Then after a short run the keyboard stopped working (PS2 connection) and subsequently everything died. M/c will not power up but as soon as power is connected to the rear of the power supply, the cpu fan runs and the cd-drawer will operate. H/disk is not running. Depressing the start button has no effect what so ever? I would assume this is a mobo failure (?) but what else is likely to have gone down. I do know the hardisk runs if removed from the system so presumably that's ok. Comments appreciated please. I can buy a suitable motherboard (socket 478) but the processor doesn't seem available now and if that is gone, is there a future for the new 478 board?

Probably mobo but have you tried powering up with the modem unplugged? DerekW

... in any event I'd avoid using that modem again. DerekW

The smell may not have actually come from the external modem, or if it did, it is a lot more likely it burned up because your power supply is failing and is putting out too much voltage than because the modem is faulty. Defective or failing power supplies are common A failing PS can partially work yet it won't let the computer boot past a certian point. Check your PS. See response 4 in this: http://www.computing.net/hardware/w...

Thank you everyone for responding! The critical factor here to my mind is that the cpu fan runs even when there has been no attempt to start the pc. That it shouldn't do, nor should the drive drawers operate before the pc has been switched on! This makes me wonder whether the electronic circuitry on the mobo which tells the power supply to start up when the On button is pressed has shorted out. (I have tried removing the connection to the on button - makes no difference). Pressing the on button does nothing at all. I have tried with all ancil boards and modem removed; again no difference in behaviour. Does this narrow things down?

My mobo does exactky the same thing! When I plug the mains power supply in the fan starts to work with out even going near the power switch at the front. Again as in my own thread when I use an old/different mobo everything is fine. Incidently, will GBW's cpu and ram be affected by this. Kind of concerned as I have pretty much the same mobo issue.

Failing or dead PS's are the most frequent thing that cause problems like yours that we hear about on this site. A failing PS can produce all sorts of bizzare symptoms. - holding the power button in may or may not work - pressing the power button may not work - it may partially power up when you merely supply power to the PS. See the info I pointed you to. Try another PS if you can. If the PS is faulty and you wait too long to replace it or it fails comlpletely, unfortunately the damage a failing PS can do to your computer and anything connected to it is random, and replacing the PS may not be the only thing you need to do to get your computer working normally again.