My worksheet has 16,000 rows and 10 columns. (Row 1 contains the
column headings).

It takes only a minute or two to get subtotals of the cells in the
selected columns. (The worksheet -- with the subtotal rows --then has
49,000 rows).

To the left of the spreadsheet a
'1' (which displays all rows)
'2' (which displays only the subtotals)
'3' (which displays the Grand Total).

When I click '2', "Calculating Cells: 100%" is displayed for HOURS in
the Status Bar.

The Windows Task Manager shows that "Microsoft Excel" is "Running";
the CPU Usage = 50%; the Commit Charge = 318M/5984M; the Mem Usage for
Excel.Exe is 64,780K; the Physical Memory (K) readings are Total =
2095188; Available = 1559000; System Cashe = 512216.

(Ending all non-critical processes doesn't speed up Excel or allow me
to run any other applications on my PC).

What's causing this situation and what can I do?