If you select all the cells with the row/column heading to the left of "A" and on top of "1", Then click on format cells and the protection tab, uncheck the "Locked" value then click ok. Then go to the certain area that you do want locked, select these cells and change the "Locked" value back to checked, then protect the worksheet. This leaves only the cells that you "Locked" protected and all others availble to manipulate
Cells are locked by default; only matters when sheet is protected. With sheet unprotected, select the cells you do not want locked (may be easier to select the entire sheet first and then deselect the cells that you want to remain locked) and then choose "Format, Cells, Protection" and uncheck the box that says "locked". I believe you can insert a note into any cell at any time.
I've tried it and it works.