Fan revs, but PC won't start

Hi,
I've been having trouble with my PC starting recently. It began about a week ago where upon being switched on, my PC would attempt to rev its fans up to a certain speed at which point it would finally start. This would take a minute or so for the fans to finally kickstart and for my computer to boot.



Yesterday though upon starting I got a message on boot asking me to either 'set my BIOS to default values' or to 'Change my BIOS settings'. I reset it to default and my PC booted up. I then noticed that the computer time was completely off.



Today I tried to turn the computer on but it wouldn't boot. Turning it on only started the fan revving like previously but this time it wouldn't start at all. Now it seems the only way I can start the fan revving process is to unplug the motherboard power cable and then plug it back in.



I replaced the CMOS battery but to no change. The mobo LED light is on.



My Mobo is an Asus P5
CPU: Intel 5500

GPU: Nvidia GTX 460 768MB
PSU: Gigabyte 500W

HDD: Samsung 500GB



Thanks for any help,

I'm in a bit of a rut as I need the computer for hoards of upcoming exams