I have two workbooks - one is a master workbook sent to me by a customer and
I need to paste information from certain cells into an existing workbook.  I
use keyboard shortcuts to move between workbooks as well as the copy and
paste shortcuts.  In the beginning it works fine but as I progress, when I
change to a workbook, some cells are 'empty' and it takes a few seconds for
the information to re-appear in the cells. Eventually, one of the workbooks
completely locks up and I have to do an End Task.  
These are the actions I've taken to try to resolve the issue but have not
had any success:
1-  I've tried different files (so it's not necessarily this particular
master file).  
2-  I've also tried copying into a brand new workbook.  
3-  I've tried putting the information in one single workbook but different
tabs and copying/pasting between tabs instead of workbooks.
4- Downloaded latest service pack (3) and any updates to Excel 2003
5- Un-installed/Re-installed Excel
6-  Checked hardware (i have a 2.2 ghz Intel Centrino laptop w/ 4GB of RAM
and 70GB of HDD free)

Please help!