Excel 2000 clocks -- Excel 2003 works fine.
 

I developed a pretty complex workbook (20 worksheets, between 500 and 2000
rows per sheet, columns A to CZ), 95% of the formulae are arrayed {}.
The workbook has multiple switches, dropdown select menus, quite a number of
SUMPRODUCTs, OFFSETs, limited INDIRECT functions, the workbook has range
names for every input (cell/row).
The complete workbook takes 25 seconds to calculate in Excel 2003 (1.2 GHz
Pentium M), if you change all switches at once.
Excel 2000 takes forever for the exact calculation, even when I don't change
a single input, it sometimes takes 45 seconds, and freezes in 80% of the
times. The very first calculation (F9) after I open the file usually works
fine.

I'd highly appreciate some advice.