Vista - Right Alt+Delete Does Not Send Ctrl+Alt+Delete

Hello:

Virtual PC 2007 running on Vista SP1 host. XP virtual machine with Additions
installed seems to start normally, but then doesn't respond to Right
Alt+Delete. Also does not respond to select Action menu > Ctrl+Alt+Del.

Keyboard initially responds in virtual machine since pressing F8 will
display menu and allow booting in Safe Mode, et cetera (booting to Safe Mode
has no effect on the issue).

Right Alt+Enter does toggle full screen mode.

Verified Right Alt is the current host key in Virtual PC Options. It is.

- Have tried removing/adding the virtual machine.
- Uninstalling/reinstalling Virtual PC 2007 SP1.

Same result. The OS seems to start okay, but then keyboard and mouse stop
talking to the virtual machine. This is a fairly mature installation that had
been running without incident for several months.