protecting and unprotecting using a macro

I am trying to write VBA code in a macro form to accept user input into a message box only. The code is to be the only method of changing values in the worksheet - i.e. the user cannot select a cell and change the value.

How can I use the macro code to unprotect the sheet in order to change the values, then re-protect it after the calculations are complete? Is there a way to protect the sheet from user changes, but not from macro changes? (I have gotten the macro to protect the sheets, but I can't get the code to unprotect it.)