Why does a new installation of Excel 2003 refuse to accept dates?

For years, I have used Excel 2003 on my old Windows XP PC. A few weeks ago I bought a new Windows 7 PC and installed Excel 2003 on the new PC. Both PC's access my home network.

When I enter a date such as 12/22/2012 in a worksheet cell using my OLD PC with Windows XP, the date is entered as a number as it should be. The date is right justified in the cell and I can use the date in formulas with no problem.

But when I enter the SAME date 12/22/2012 a cell using my NEW PC with Windows 7, then the date seems to be entered as text. The date is LEFT justified, will not format as a number, and doesn't work in formulas.

This same behavior happens regardless of whether the worksheet itself is on the new PC or the old PC and regardless of whether or not it is accessed over the network.

There seems to be some option on the new installation of Excel 2003 that is preventing it from accepting the input of dates.

How can I fix the new Excel 2003 installation so that it accepts dates correctly like the old installation does?

Thanks for any suggestions.

Sure enough, changing the Regional and Language Settings fixed the problem. Thanks so much !!
Here’s what happened:
In XP, the “Short Date” format was “12/22/2012” (using numbers)
In Win/7, the “Short Date” format was “ddd, MMM dd, yyyy” (using letters). So I changed the “Short Date” format in Win/7 to mm/dd/yyyy, and now I can enter dates into my Excel worksheets on Win/7 with no problems.

Although I have to admit that I don’t understand WHY this fix worked or what’s really happening.
How does the Win/7 “Short Date” format affect the internal processing of Excel?
What else is affected by the Win/7 “Short Date” format ?
There is also a “Long Date” format. How does that affect Excel or anything else?
Is there an explanation somewhere of how these settings work?
Thanks for any additional information, although the immediate problem is solved.