fc5 rescue and install dvd both boot the on-disk system

[Wolfgang S. Rupprecht]

I just did an upgrade install of fc5 on an LVM partition that spanned
2 disks.  After doing that install I wanted to boot in rescue mode and
touch-up the LVM size and the underlying ext3 fs size.  Turns out that
both the rescue disc and the install disc no longer take me to the
dvd's install/boot menu, but boot the normal hard-disk system.

I think the first stage boot does come from the dvd because I see the
dvd access light flicker and the top screen line says "booting CD".
the rest of the boot stuff seems to be from the on-hard-disk
/etc/grub.conf file.  Short of clearing out the disk's /boot, how does
one get the dvd to supply the programs to boot?

-wolfgang