I'm not aware of any documented shortcut in Excel for Ctrl+Alt+Arrow Down.
That shortcut will dim the screen in Windows and Ctrl+Alt+Arrow Up will
brighten the screen up to the limit.
I would assume you assigned your own shortcut key to a macro.
BTW the use of Ctrl+A can get you into serious trouble in Excel 2003.
http://www.mvps.org/dmcritchie/excel/shortx2k.htm#foobar
What did the shortcut do for you?
[in other words what does it do instead of work?]
Perhaps you are thinking about Ctrl+Shift+Arrow Down which extends the selection.