Protect all but one cell

Select cell C5 on your worksheet.
Click Format->Cells...
On the Protection tab, un-check the box that says "Locked"
Click OK to close that window.

Now click Tools->Protection->Protect Sheet...
"Protect sheet and contents of locked cells" should be checked
Type in a password to un-protect the sheet, then click OK
Re-type in your password and click OK
Save the file and you're all set. Now when you open the file only C5 will be editable unless you un-protect the sheet with your password.