Thanks a lot for your help. That's exactly what I needed.
Yum couldn't find kernel-headers, but it found kernel-devel. After I installed this, I removed/reinstalled VirtualBox successfully and successfully created a Debian virtual machine.
I noticed a couple of things about VirtualBox:
1. It appears to be a little smoother than the VM software that came with Fedora.
2. The layout reminds me of VMWare.