Excel freezes

[DL]

Excel 2003, win2k updated, 1gb dual ram, Athlon64 3200+, swap win managed,
oudles of free space

I have a custom workbook provided by a software house (in use by many users
over several years) that contains macros/functions/addins I frequently have
problems with freezing (excel only), particularly if 2 workbooks open.
Another example; if open directly from a saved workbook there is frequently
a problem, but if I start Excel then open the saved workbook its ok.
No other Office, or any other apps, exhibit this behaviour. The software
house believes its a problem with win2k, they use winxp, does this
explanation seem logical, if not is there any other avenues I could explore?
(i've run detect/repair)
TY

 

[DL]

To add the freezing only occurs in Excel when using a workbook/sheet with
data that utilises the custom features. Using excel with std data doesnt
cause any problems.
(The customisation is an Accounting feature)