HP Pavilion 7940 wont boot or p.o.s.t.

Hi. I am looking at a mates pc which is having the following problem. When its turned on, it shows the Hewlett Packard blue screen and does not boot any further, not even as far as p.o.s.t. There are no beeps or anything. All the fans are working and the dvd/cd drives will open. Numerous re-boots and the same thing happens. Now, when the machine is unplugged and left for say a few hours, or overnight, it will then boot up into windows with no problems! If re-started fairly soon after this, it may boot up again no problems, but if left for a while and re-booted, it will hang at the HP screen again and wont p.o.s.t.

I have replaced the mobo battery and re-set cmos. I have also stripped the pc and cleaned everything including the mobo. Any ideas on this problem?

Its a Hewlett Packard Pavilion 7940 with an Asus A7V-VM mobo (not sure which variant), running XP Pro.

 

There could be a short on the motherboard or the PSU could be dieing. The BIOS chip could also be failing.