Select the whole worksheet, (use the square between the row and column labels at top left). Right-click, select Format Cells.. and then the Protection Tab. Clear the 'Locked' check box, click OK.
Select the one cell to be locked.
Right-click etc. and check the 'Locked' check box. click OK.
Now apply password protection to the whole worksheet:
Tools - Protection - Protect Sheet.
You can accept the default or you can clear the 'Select locked cells' check box.
Even if the user can select your locked cell, they will not be able to change it.
Add password and follow remaining steps.
You didn't say what version of Excel you are using.
The above is for Excel 2003, but a similar approach should work for Excel 2007. There isn't a Menu with 'Tools', so if you have Excel 2007, you will have to find the worksheet protection dialog box from the Ribbon. I don't have Excel 2007 on this PC, so I can't walk you through it.