I have a HP Digital Entertainment Center Z552 with XP/Media center from 2005 originally with 2 pieces of 256 MB RAM. I added 2 PC 512 MB and started the computer and it worked fine, the processor fan run slowly. After a while the computer went in to standby mode due to no activity as it normally does. Next day, when trying to start the computer again pushing on the power on button the processor fan run fast and nothing else happens. The power button turns red (standby) instead of blue (normal operation). The screen is blank. I have checked the led in the back of power supply unit and it green lightning. If I press the power button a while the computer shuts done but the starts again after approx 5 seconds in same state as earlier, i.e.. power button red, processor fan running fast and blank screen.

I have tested to remove all RAM, then computer is dead. Tested with only one and only two RAM, same procedure. Last thing I tested with the old RAM same thing and finally tested only with the new RAM:s.

Any idea what has happen and how to fix so my old computer starts up again and running even better than before!?

My guess would be that your power supply needs to be replaced.

Ok, but the the fans are running high speed and there are text on the front display saying "welcome to ...".

Do you mean that the power supply can be broken but still send power to some parts in the computer but not to the "vital" parts?