Pop up alert message to fill the empty cells

Hi All,

I am not so good at VBA coding, so can you help me with this.
I have a question: In excel while we fill data if user skips to enter data in any cell can we pop up any alert message which guides user to fill up empty cells before they proceed to any other cell?

I need alert message for certain columns from column A to I,
R to S,
only U,
W to AC.

and in Column S if we skip to enter data we can either enter in column T
viceversa if we skip to enter data in T should enter data in column S.
i.e data should be entered either in S or T, Cant leave both columns empty.

I need alert message for the entire cells not for a particular range of cells.

I need a alert pop up message soon after the user tries to skip entering data in any cell in excel2003,Using VBA coding is this possible?And i dont require any validation for this.Please suggest any solution with coding.
Solution to this problem would help me a lot...

Any help would be appreciated...
Thanks in advance...
akki83