PC wont power up long enough to boot up

Sorry if there is an answer for this somewhere already but I cant find it.
Basically when I switch the tower on the PSU,CPU and graphics card fans all spin up and then stop after about 2 secs the master HDD also spins up but also stops nearly immeadiately.
The problem started after I turned the system of because I was going away for the weekend, there were no apparent problems with it before.
I have tried swapping out the PSU,RAM and graphics card, I have cleared the CMOS, reseated all power cables and cards tried disconnecting the FDD,HDs and CDD in turn and all I get is the same results.
Any ideas would be welcome it is driving me insane at the minute.
Cheers
steve

 

Did you try replacing the CMOS battery ?

 

No I have not yet as it is only about 10 months old, and I didnt think it is staying powered for long enough. But I will give it a go anyway nothing to lose!!!

Tried replacing the battery with a new one still the same problem, I dont think it is getting to POST let alone BIOS or CMOS.

 

Disconnect all but one drive, one stick of ram, and keyboard and monitor, and see if ir posts. If not, suspect a dead motherboard or psu. Substitute psu to test. Otherwise (if it posts) add things back one at a time and test.