Compile error on Excel 2010, but not on Excel 2003

I can't explain what is wrong with this simple code. It works perfectly on Excel 2003, but in Excel 2010 it displays Compile error: Sub of Function not defined.

Can anyone explain why is there an error on Excel 2010. Thanks.

   Sub Test_File_Opened()
       
       If IsFileOpen("D:\Test.xls") Then
           MsgBox "File is open!"

       Else
           MsgBox "File is closed!"
          
       End If
   End Sub