Here's an odd one. Pc freezes-
 

Here's an odd one for you guys out there.
I have 2 pc's with a KVM switch. One a dell and one I built a few years ago. Both are P4 3.0
My non dell started to freeze to the point of having to reset. It would then work for a while - minutes or hours on the odd occasion. So I thought could be the graghix card so I borrowed one and installed it - but no still the same. Then one day I was doing a search for a file and it froze but the search was still working, ah I thought the kvm switch so I unpluged the mouse and plugged it in to a usb port - but no still nothing. And the same with the keyboard. Next I did a memory test that was ok. So I thought maybe the mobo as I had the sata ports fail on me intermitently so I had to put in an ide hard disk. Or maybe it's the hard disk as usually when it freezes it's accessing it. (except for the time I was searching). So I had an old 40 gb ide so I put it in and installed vista. Other pcs have xp. So I booted up into vista. Now what happens is the mouse and keyboard stop working - but if I unplug them from the kvm switch and plug them into usb ports directly I can use the pc. Thinking it might of been a cable fault I swapped the kvm cables round from dell to other and vica versa. But it still does it.
Whew there you are. Any one any ideas?

My sister and her boyfriend have been complaining for about a year that their PC (my previous PC, built by myself many years ago) sometimes doesn't boot up: when switching it on, the hardware starts working (hard disks and fans spin up, LED's light up) but nothing appears on the monitor and, apparently, the OS doesn't get loaded either. Pushing the RESET button usually helped but a few weeks ago even this was not enough to make the thing work properly.

The problem is not entirely unlike yours and the reason was very obvious: the (relatively) large, (relatively) high-voltage capacitors on the motherboard were puffed up a bit: their top was not flat but visibly round. (Well, 4 years of pretty much constant usage.) I was brave, ;) went to the electronics shop, bought capacitors of the same parameters and carefully (!!!) soldered them in place of the old ones. To my extreme surprise, the thing works now. (Except for the PS/2 mouse; probably, the capacitor near its port is not soldered properly, gotta check it when I have some time.)

So, if you suspect the motherboard, check the large capacitors.