Well, i am not a developer or a mac-genius and i need help!

This morning i worked for a few minutes in final cut pro 5.1 (on my iMac 24 - 2.16GHz Intel Core Duo with 1.5 GB 667 MHz DDR2 SDRAM with OS X 10.5.4). Then I shut down the mac, took my external Harddrive to a friend, copied some files, came back home and then....

my mac was not able to start up again. after a while i managed to start it in secure-mode by pressing shift. when i tried to restart it from there, it stuck again. i found out, that my quartz extreme is disabled and it is usally not (final cut pro does not work when QE is diabled). My computer sais, that my grafics card does not support quartz extreme... but thats not true... it was working this morning... my graphics card is an NVIDIA GeForce 7300 GT with 128MB VRAM..
however, i think that these two problems belong to each other... if my quarzt extreme was working, my mac would start... (hopefully) can anybody tell me, how to enable quartz extreme in OS X 10.5.4 ?

thank you,

kevin