No booting but fan runs problem
During the week I had a problem where on switching on my computer it would not boot but just the fan run and the motherboard light would also go on and the LCD monitor would just say "No video input".
At first I suspected that one of the cables on the motherboard had got loose but on checking every one of them the computer would still not boot even when I tried to boot using the Ubuntu Live CD.
A week earlier the same thing had happened but after a few attempts at starting the computer it finally booted but this time it did not start at all.
I got some very useful threads on TechRepublic and so I went on to follow some of the suggested ideas.
In my case I realised that the problem was with the power supply unit. I took a power supply unit from another box and tested it on the computer and it booted straight away and so I replaced the power supply and the problem is gone and the PC is now booting as usual.