Wierd problem with my Dell Dimension 2350 PC

I got a Dell Dimension 2350 that on pwr up it changes states back and fourth from the POST screen which displays the Dell logo and type of PC model and then displays that it is trying to load Windows XP then it goes back to the Dell Logo and then again try`s to load Windows XP. It keeps doing this back and fourth for about 2 to 3 min and then it finally stays displaying on the screen Windows XP like if it was loading it but it does not it just stay ther doing nothing,also the display screen goes dim. I did a clean installation of Windows XP thinking that perhaps the Boot up files were corrupt, the installation was successful, but when i booted up the pc it did the same thing again. I even replaced the HD with a spare that i had then successfully rienstalled Windows XP on that HD and it still did the same thing. So obiouslly the problem is not with the HD or the OS. Has anyone seen this problem before? Could it possibly be a failing motherboard? Help im lost!