The information here will help you reset the used range:

http://www.contextures.com/xlfaqApp.html#Unused


Mark wrote:
I have several large (as in using a vast number of cells) spreadsheets that
are becoming a problem regarding their size. When I hit ctrl-end to see what
excel recognizes as the end of a particular spreadsheet, it, for some reason,
includes an additional 500 rows. How can I get excel to recognize the actual
last row/column cell?


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Debra Dalgleish
Excel FAQ, Tips & Book List
http://www.contextures.com/tiptech.html