It also didn't let me upgrade the boot loader (I was using grub). But I choose to install a new one (still grub).
In the installation option, by default it offered me my 2 partitions (fc and vista) and I just had to change the labels.
But once installed, I found my new grub had 3 entries, the default one was pointing to the old fc6 kernel and was not working because it couldn't find the file. But there was one pointing to the fc7 kernel that I could manually select.
So I just edited my etc/grub.conf, removed that fc6 entry and made sure the one pointing to the fc7 kernel was default.
Not sure what's going on, but you may try to restore your /etc/X11/xorg.conf file from a previous version or run X -configure