Excel VBA to force cell entry if another cell is not empty

i believe somebody could help me with this excel spreadsheet that i'm trying to develop. i just don't know how to write the codes and where to place those. i want to force every user of my worksheet to fill say cell b1 if a1 has entry in it and they cannot move to another cell if cell b1 is still blank. i am also trying to activate the code with cell b2 if a2 is not empty. Please also make the code run even the worksheet is protected.


Thanks for all that may help.

cedric[/B]

Thank you very much Sixthsense for the solution. this problem keeps my eyes and head aching for days now. Your help is very much appreciated.