You can't protect a particular cell only - you have to protect worksheet. But you can determine, how the protection will affect particular cells in this worksheet (is editing allowed, is data validation allowed, are formulas/manually entered values displayed in formula bar, etc.

Select whole worksheet (click at left of 1st column - column A - label). Select Format Cells and activate Protection tab. Set options Locked and Hidden for all cells on worksheet;
Select any range in worksheet you want to follow different protection rules. Select Format Cells and activate Protection tab again. Set rules for this range;
Repeat the process until you protection rules for all needed ranges are set;
From Review menu, select Protect Sheet. At bottom of Protect window, you can allow additional options for locked/unlocked cells in worksheet. Enter password and click OK. you are asked to repeat the password. Optionally you may leave password unfilled.

After you have protected the worksheet, user can edit cells which are unlocked, and he/she can't edit any other cells in worksheet.

There are some protection options, which you can set only using VBA code (I'm not sure about Excel2007, but in older versions the option to set additional allowances when protecting the sheet was not available - you had to do this through VBA only).