Booting up sometimes

i have an asus a8nsli deluxe running a amd fx processor, sli nvidia 6800 cards and coarsair ram. when i first boot up i havent been seeing the bios (sometimes) then after the win xp screen it stays black...I can tell that it continues to boot, but the screen stays black. I force restart and it does the same thing. I manually shut it off via power supply switch then go through the start up process again and it most likly works. sometimes i have to do this a couple times for it to turn on, then when it gets to the desk top it takes forever to get to a point where I can run programs. the computer was working fine about a week ago and then this suddenly started happening. I'v updated the drivers for the graphics cards, but i think the last windows update screwed things up cause it seems like the problems came after the update but I'm just not certain of that....anybody have any suggestions?
thanks