In Office 2007, Click the Office button and click Excel Options which you will find at the end near Exit Excel.
Go to Formulas tab and under Working with formulas, Check or uncheck R1C1 reference style to use it or to change it to A1 reference style.
If it doesn't stick you have some problem with a personal.xls
or the default template or something I guess...
EDIT:
Try this first:
Close all spreadsheets down. Assuming you have created one previously, unhide your Personal.xls
workbook (Window>Unhide) and then uncheck the R1C1 reference style. Save your Personal.xls, rehide & close down Excel (clicking Yes to save changes to Personal.xls). With any luck you should now have your default A1 style back. You will have to repeat the above with any other workbooks in your XLSTART directory and/or the start up location in Tools>Options>General tab.
Then this:
Help > Detect & Repair.
If no good, try http://support.microsoft.com/kb/291288
Last resort:
Use Start > Run excel /regserver
NOTE the space after "excel"