Column header is numbered 123 instead of ABC in Excel 2007

By adjusting a setting in the Excel Options dialog box, you can revert back to the default setting of column header which is ABC...
To change the setting, perform the following steps:
1. Click the Microsoft Office button and click Excel Options.
2. In the Excel Options dialog box, in the left pane click Formulas. In the right pane, under Working with formulas, click to remove the check mark besides R1C1 reference style.
3. Click OK to close the Excel Options dialog box.

Excel 2007 error

Error message: The operation you are about to perform affects a large numbe rof cells and may take a significant amount of time to complete. Are you sure you want to continue? Note: This operation will be performed automaticaly in 60 seconds if no response is received.

Solution 1: Press CTRL + END keys from the keyboard to know the last cell that contains the data. If the valid data is available till column E and the last column that Excel navigates when you press CTRL + END is XFD or whatever then you have to click the column header F and continue scrolling to right till XFD. Once the selection is over, right click the column header and click Delete. Save the workbook and close and reopen the workbook.

Solution 2: Try clearing the temp files on your computer START >> RUN >> %temp%>> ok Select all and SHIFT +DELETE Empty recycle bin