Excel 2007 Slows Spreadsheet Navigation

Several of my users share an Excel workbook. Up until a couple of weeks ago, they all had Office 2000. Then, one was upgraded to Office 2007. Ever since that user opened and saved the workbook (in 2000-2003 format) the navigation and scrolling, etc., one of the worksheets is SLOW! This is for the 2007 user as well as the 2000 users. I have done some testing, including restoring the file and making copies, and pretty-much confirmed that Excel 2007 caused the problem. Also, I have noticed that saving the workbook using 2007, still saving in the 2000-2003 format, can increase the size as much as 50%. HELP!!! Any ideas??? Thanks!