Hi, sorry I'm a little late getting to this, I see Mike replied below.
In case you need more help: to record the macro, go to the Tools Menu,
select Macro->Record New Macro, and name it Workbook_Open. Start
recording, and change the zoom to 80%, (in fact, make any other changes
you would like at startup), and then stop recording. Once you have done
this, go to the Visual Basic Editor (Alt + F11), and your macro should
be there in Module 1 in your workbook. Copy the whole thing from Sub to
End Sub, open up the ThisWorkbook object under PERSONAL.XLS in the
Project Window, and paste the code in. Then you should save, close and
start up Excel again - the macro should run and zoom automatically.
I would encourage you to experiment with macros, even if you are not
confident with them - they can make working with Excel particularly much
easier. Have a look also at
http://office.microsoft.com/en-us/excel/HA010346281033.aspx
for a (slightly different) explanation of how to create a startup macro.
Good luck