Does anyone have any thoughts on how to arrange this in Excel 2003:

I have a group of invoice numbers.
I have columns of inforamtion relating to the invoice: inv #, total amount,
inv date, etc.
That is currently set up with the numbers in the first column and the rest
of the information in the columns next to it. So, if I have 1000 invoices, I
have 1000 rows of data.
Now I need to make payments against those rows. Each invoice can have
anywhere from 1-20 payments made against it. For these payments, I need to
record the payment date, the # hrs being paid, the amount being paid, and the
payment number (1-20).

Does anyone have any thoughts on how to arrange the data without having 4
columns for each of the 20 payments totaling 80 columns? I need to be able
to report and pivot off this data. Thanks for any suggestions.