|There are many possibilities that could cause this problem.|
Did you used anti-static wrist strap while assembling the computer? Or at least statically discharge yourself before assembling?
Did you properly installed every components?
OK, I assume there are no faulty components and all components are properly installed, then maybe the problem is BIOS error, then you need to reset the BIOS. You can read detailed instructions here: http://pcsupport.about.com/od/fixth...