F8 will only function if the system can find the operating system to begin with.
This is more than likely the result of:
a - an aggressive virus.
b - a hard disk failure.
The windows disk should load regardless of either of those possibilities though. Since you have only filled out your operating system in the system specs portion of your profile here, I can't give anymore direct instruction. I can only assume the windows disk is not launching because the hard drive has been set as the first boot device in the bios. Without knowing more system specifics, I can't give detailed instructions for fixing that problem.