Failed win7 installation, black screen on boot

I have an Imac, about 4 years old, 2ghz core 2 duo.

Last night i attempted to install a copy on windows 7 in bootcamp. I created an install DVD, succesfully created a partition and got about 1/3 way through the installation when it failed. I tried twice then shut down the computer. Today i started the imac, boot sound worked, then after white screen for a few seconds, black screen with a flashing white - in the top left, like dos. I'm unable to type anything.

I've tried booting in safe mode, selecting boot device etc, but nothing. the only thing i've been able to do is eject the windows disk by holding f12.

I'm using a logitech wireleskeyboard and mouse if that matters.

I have also tried creating a bootable usb drive with mountain lion,but as i cant get to the boot menu, it's no use.

I've got all my dada backed up externally so i dont mind having to freshly install if that's an issue.

any ideas? thanks