It could be a (internal) component or (external) device that caused pre-POST hung.  Remove external USB drive prior to boot up if you use any.  For internal, remove everything except CPU and re-install each component at a time for each test.  Use integrated VGA if possible during test.  Does F12 provide menu for BIOS update option via USB.  Try the RTC reset if you haven't done so.
I have a Dell Precision T1700 Workstation. The problem is that every time i try to reboot or restart the Computer the display stays Black. So in order to get it work again i need to replace or add a Memory Module. If I do so the computer does start with the message " Memory has changed" and then it boot normally. I try to flash the Bios with the latest version (A28) but since the computer needs restart it can not be completed!   Any suggestions? (cmos battery is new)
dell precision t1700 workstation