Thanks, everyone, for the responses, but I agree with Dal in that neither solved the problem. However, I found the answer on the TechNet forum. To restate the problem:
When starting a virtual machine in Windows Virtual PC on Windows 7, a progress bar is displayed rather than the DOS boot screen. I need access to the DOS screen during boot so that I can bring up the Advanced Boot Options screen. The progress bar display does not allow me to do this.
While the answers above regarding the F8 and Del keys are accurate in how they affect boot up, both require that the DOS window being displayed, and getting to the DOS window is the crux of the problem.
Which brings me to the answer:
Pressing Shift+Esc when the progress bar window is displayed switches to the DOS window where the F8 and Del keys may be used accordingly.
Thanks again to all for the responses and for making this a better community.
PS - smjain posted the answer here.