Sounds like one of those Windows update situations.
Not sure yet that is what has happened, but I do not have a solution for the update problem. The folks that did recover from such a problem used the retore to previous option.
Maybe someone else know of a solution, IF this is what happened.
You should be able to repair it using the DVD or CD, but if F8 doesn't help, try tapping the space bar during boot. Maybe it will get you to a Advanced boot options menu.
If it really is a hardware problem, how much memory do you have installed and can you remove some?
Insert the install CD and then reboot go into setup (F2 or F10 depending on BIOS watch the boot screen for correct key) and select boot from CD (first boot device) go through the menus untill the install screen. But, goto recovery console and the ASR should start automatically or you may have to press F2 to go through the Automated System Recovery (ASR). This will roll back any update problems. Chances are you next boot will work properly. Good Luck!
Just in case, if this problem is related to the Windows Update, I would turn automatic updating off. Then just allow one or two at a time to see if problem returns. I never allow a hardware or optional update without checking it first.
If the problem is recurring, it might be something corrupting your system, like memory, or bad drive, or even heat. If you can get into your system, unplug it and maybe re-seat the video if it is an add in card while you are dealing with the memory. I assume you are not overclocking...