It's your PSU.
Quote:
I have a self built PC that is not booting, I know the power supply works, as all the fans and lights turn on,
No...you don't. All you know is that your PSU is able to turn the fans and lights on. It's your PSU. All your comments suggest this:
*It's 3 years old (EDIT: I may have made this up...I just re-read the post and it only mentions last Xmas. Still, it's your PSU)
*fans and lights turn on - very typical of a failing PSU. Often makes you think it must be something else because, well, fans and lights turn on.
*I've learned from experience, THE most likely component to fail is the PSU.
It's your PSU.
Hope this helps.