OK, this isn't really very intuitive, so pay attention.
In Excel all of the cells in a spreadsheet are protected (locked) but the worksheet itself is not. So you need to do two things to protect your formulas.
First, highlight the cells you DON"T want to protect - the not-formulas part of your worksheet - then right click and choose Format Cells. Click on the Protection tab and uncheck Locked.
Second, from the Excel menu bar choose Tools, then Protection, then Protect Sheet. You can enter a password to unprotect the sheet, but usually it's not necessary. Click OK.
Your formulas are now protected. To make it easier to differentiate between the protected and unprotected areas of the worksheet I usually change the font color in the unprotected areas. I like a nice blue.