OS= XP Pro SP2 all updates in.

Just installed Office 2003 with SP2 update. In Excel I want to use
dates in the month-day format (Oct-20), so I highlighted my date
column and used Format/Cells/Date to apply the month-day
format. The dates format properly, but all days revert to 05,
i.e. Sep-30 enters as Sep-05, Oct-10 as Oct-05, etc. What could cause this?
Tried a blank cell in a new sheet with no change.

If I use Format/Style/Modify/Date, the dates enter correctly, but
all 5 digit numbers in the sheet change to dates, obviously the
date equivalents of the numerical values. How do I stop this?