How do i protect a single cell with a formula in so that it cant be change or deleted?
There are two steps.
1) Right-click on the cell in question and choose format cell. Select the protection tab and ensure there is a check in "locked". If locked is selected then the cell will be protected after step #2 (by the way, locked is default for most versions of Excel).
2) If you have2007 or higher then select the "Review" ribbon and click on the "protect sheet" icon. The default settings are adequate here (everything is disabled except the ability to select the cell). You can also protect the entire workbook instead of a single sheet - up to you.
If you have an earlier version ofthen you would click on tools from the menu and then select protection. From there the steps are identical.