Computer won't start

I've got a Dell desktop pc around 7yrs old, that has been running ok till a couple of days ago and now I can't start it , the power light is on at the rear but tried different leads etc and completely dead, no fan running or anything, I'm not a computer buff so any ideas would be welcome

Most likely the power supply is broken, if its not that then its the motherboard or the wires from the motherboard to the switch or the switch is faulty.

funnily enough i had this problem, it was a dead videocard, when i removed the videocard it fired straight up, with it in it was completely dead

Depending on which Dell model it is, it should have four diagnostic lights either at the front or rear, marked A to D or 1 to 4

These will light up / flash amber or green depending on hardware status.

What colour are the lights?

What model of Dell is it?

If it is the power supply, then something you could try is to remove the power cable and hold the power button in for ten seconds (the power light may flash briefly) then plug in the power cable and turn it back on.

I've done this trick with Dell Optiplex desktops and Precision workstations over the years. . .

Try removing the RAMs and change up the slots if Mozalans idea dont work, it could be anything from dead psu to rams or motherboard or gpu, so te best thing is to try individual components in different setup to check which component is failing

Thanks for all your responses, I've tried pressing the power button etc though no success, not taken the cover off as yet. PC is a Dell Vostro 220

there's a couple of links here regarding Dell Vostro computers with similar problems:

http://www.tomshardware.co.uk/forum/id-2097777/dell-power-problem-amber-light-flash.html

http://en.community.dell.com/support-forums/desktop/f/3514/t/19381100

The second link tells you status of the computer via the power LED - definitely worth a look.