I've got a Dell Inspiron 1440 laptop that suddenly broke down a few weeks ago.

The screen turned black when powering on and the laptop started the booting process yet the screen remained blank.
Then it got stuck in the bootsequence. I expected the motherboard to be damaged or maybe some part burned out.

I've ordered in a new mobo from the US and built it in;
when I power it on now everything lights up and I can hear the DVD drive make its usual clicking noise (it didn't do that with the old mobo),
after a second or so the laptop reboots again, and again, and again.. it seems to be stuck in an infinite looping reboot.

Here's a video



If I remove the memory modules I hear NO (!) beep.
The CPU seems fine to me because without a working CPU the bootsequence wouldn't even come up.

Maybe I've missed a cable or connected something the wrong way?
Not too sure what to look at, I've followed the instructions precisely in taking it apart and reconstructing it.

Any advice?