I found out recently that you can easily protect just a few cells in a worksheet, 
without having to go through the nausea of unprotecting the cells you want to keep 
and then protecting the whole sheet.

Use Data-Validate for the cell you want to protect, with a range of values which 
are the same as the cell's current value. Eg if it is set to 0.5, the range is 0.5 to 0.5. 
This protects the cell. You can even have a rude message coming up if someone attempts to change it.

Nigel.