monitor only usable as desktop extender?

hi, i've been having some trouble with my 17'' lcd. here's what's goin on.

i have a laptop and i want to use my lcd flat panel as my primary monitor. i did this successfully (although i wasn't able to get the resolution to the monitor's native res), but as i was trying to fix this resolution problem, i clicked "extend my windows desktop onto this monitor" checkbox that is in the display properties. ever since then, i am not able to use the lcd flat panel as anything other than a desktop extender. that is, i can't use the thing as my primary monitor. if i uncheck the "extend my windows desktop..." box, i get a blank screen on the lcd.

i don't understand how it worked before and now it doesn't. anyone know what is going on?