strange power issue - help needed

Wasn't sure what to clasify this problem as but desperately need help.

Came back from a 5 day holiday and tried to switch my PC on. The PC took about 15 seconds before the power kicked in!!

Now this is happening all the time. I opened the case and then hit the power switch and imediately noticed a little LED on the Motherboard go on so i can only assume that the Switch is fine.

I remembered that the CMOS battery was loosing the time before i went on my hols so i hoped that some checksum or other may have been causing the PC not to power correctly - but after replacing the battery the problem is still occuring.

Once the PC has booted it never has any issues with the power supply.

Worked with PC's for some time but i'm stumped with this one.

Any help or comments appreciated.

p.s the PC is a Shuttle.