What are you doing when you get the error? Are you copying or filtering data? If so try putting the Excel App into Manual calculation mode first and only calculate after the process.

What sorts of formula are you using? If you have lots of array formula it's possible these are taking lots of resource. What version of Excel do you have? (Incidentally when you get a moment would you add that information to your profile so that it shows in the post) It's useful to know since there are be more friendly SUMIFS & COUNTIFS formulae available in XL 2007-->