alternate sort on different columns

[Revontulet]

I try to sort a spreadsheet (Excel 2003) alternatively in two different ways,
independent of each other.
It means, if a certain condition is met, the data should be sorted on column
A, if the other condition is met, it should be sorted on column D.
Right now I have the problem that Excel remembers the previous sort (that
is, if I first sorted on column D, the next sort on column A keeps the
previous sort on D - which I don't want.
How can I undo the previous sort?
Aurora