starting up my computer press button on tower everyhing runs as normal ie screen messages and when the window logo appears and the oading bars at the bottom of the screen stops it cuts out and tries to restart again

ken, December 2010

Another possibility:

Your CPU is overheating due to a dust clogged heat sink.
As the CPU heats up, it slows down and finaly give up and shut down.

Clean the heat sink of the accumulated dust.

Electro, April 2011

Had a similar problem with a tower recently. The Power Supply was OK when cold but as it warmed up it failed and sent a signal for the machine to shut down - which it did - and the power supply cooled down and began to work etc.

I bought a second hand PSU for £20 - a new one would have been £40/50.

Peccavi, December 2010