Hiya becsin
Firstly select all the cells and go to "Format" then to "Cells..."
Then click on the "Protection" tab and uncheck the "Locked" box and click OK. This will allow people to change stuff when your worksheet is protected.
Next, select all the cells you DON'T want people to change. Go to "Format" > "Cells..." > "Protection" tab. Now click on the "Locked" box again. A check should appear in the box. Now click OK. This makes sure that the cells you wanted to protect are protected.
Now go to "Tools" -> "Protection". You have to do 2 things. Click on "Protect Sheet". Put a password, if desired, and click OK. Then click "Protect Workbook". Again put a password, if desired, and click OK.
Your cells should be protected from being overwritten now.
HTH
coyotem78