my cd tray opens on start up

I have found a PRAM reset usually cures the problem.
Hold Command Option+PR while booting.
Continue to hold the keys until the second startup chime, then release.
The machine will then continue to boot.
http://support.apple.com/kb/HT1379

At what point is it doing this? How long does it stay open? How long does it take to boot up your computer. I am wondering if your computer PRAM is corrupt and the computer doesn't know from which drive to boot so it is offering to let you put in an optical disc for booting. If that's the case perhaps your PRAM battery is weak.

hi
the first thing to try is another keyboard

hi
you could try removing any usb & firewire devices except mouse and keyboard
to see if the problem is there.
you could also try zapping the pram.