my mbp wont boot with external display connected!!!

Hi all,
running a 15" mbp 2.5ghz ( recently invested in 24" Dell 2408WFP and wireless apple keyboard )

I'm unsure what the problem is, but when the display is connected and I turn on my mbp, it hangs on the light blue screen before the desktop shows.
I bought the new screen and keyboard on the same day, and the problem has been happening ever since so im unsure if its the screen or keyboard causing the problem..

I need help!
Thanks

 

ps. the problem does not exist when running in clamshell mode. only when running dual displays does my mbp not fully boot.
thanks.