Desktop Startup Issue - fans on but no boot

Ok, I'm new to these forums but this site has been a great help to me over the last couple of days so hopefully you guys can help me out here.

Before I start just a quick background, I've been fixing computers for years without any problems but this one has me stumped - and it's driving me mad that I can't fix it. I have seen a couple of similar posts to this but I've not read any solutions yet so here goes...

I had an old barebones system that was being resurrected for use as a second PC. It consisted of mobo, PSU, CPU & fan, HDD & DVD. I had to buy some new memory and AGP card to use in it, which I got some PC2100 and a GeForce FX5500. I started having some issues with a virus as i had to go online to download XP SP2 and so was fairly vulnerable to attack. The virus was getting worse and therefore soon after this I decided to reinstall windows on the PC, however before I got that far it started to have power issues where it would just not switch on. I would press the power button on the case and the CPU fan would come on however there was no display/boot/or any activity. It would turn on and then I could not turn it off via the case switch - I had to switch it off at the surge protector since the PSU had no switch. My first thoughts were that the Mobo/processor were dead.
I then bought a new Athlon XP CPU and fitted it into the motherboard - still no response. I bought a new PC Chips mobo and fitted the new CPU onto it - and there was some life. It would sometimes switch on and allow me to reinstall windows, other times there would be no life and it would exhibit exactly the same symptoms as before - except the case switch WOULD allow me to turn it off. For each time the PC would would not turn on I would go through the procedure of removing all unnecessary components from the mobo, HDD, DVD Drive etc, turning off power at the PSU, at the wall etc. I also would reset the CMOS and this sometimes seemed to allow it to switch on, however this was not always the case.
So finally I bought a new PSU unit and fitted it - unfortunately this did not help the situation much and the symptoms remain. I can still switch off the PC from the case switch when it does not start. Sometimes it allows me to boot up and use the computer fine. It went through a stage of working providing that I would switch off the PC rather than doing a restart, however as before it did not seem to always be the case!
I am stuck and cannot work out what the problem is, however there are two areas I am going to look at next. The PC is running off a long power lead, I have replaced this with the most reliable one I can find but to no avail. The other is the case - it is very old and I thought there may be an issue with the switch - I have tried changing over the power and reset switches but that also has had no affect.
I therefore have a PC with new mobo, CPU, PSU, memory, GPU, and still having problems!! Does anyone have any suggestions as they would be much, much appreciated!

 

Well, I would remove the BIOS battery, take away all power and set the CMOS_CLR jumper to on ... obviously if you use an UPS remove the cord going from the UPS to the computer. Now remove all HW components, all unnecessary cards, including graphics (use onboard if you can), remove everything, INCLUDING case fan, front USB and audio ports, Optical and Floppy drives, all HDDs, internal speaker, absolutely everything, leave on CPU, CPU fan, a single RAM chip. After removing all of these components, replace the BIOS battery with a new one, set the CLR_CMOS jumper to default and try to boot.

 

Since you have also replaced the mains power cable that only leaves the monitor and monitor leads. Try it without the hard drive first.

It is also possible to receive a duff new motherboard. We have had posts where members have had two or three in a row!