This sounds like a serious problem - I cannot imagine any reason for a workbook to use up so much memory. Are you sure you are interpreting the task manager data correctly? Can you say what is the actual figure you are reading (before and after opening the workbook), and for what kind of memory?
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I think I make no mistake interpreting the task manager. maybe its becaus of VBA which uses quite big memory? but anyway, the usage is maximum as soon as I open the workbook...
I continue expertizing and seem that it happens because of my macro. so the problem is how to free memory after the VBA macro finishes work ?
It's hard to say without knowing more specific information about your vba program and how it works.