Nik,
Conditional formatting is pretty easy here, but would only be a reminder to the user to fill in the highlited cells
using two arguments I beleive that if you format cell 2 to be "red" when cell one has data or a value greater than 0, then have cell 2 turn back clear when it has value greater than 0. you could in effect remind the user that you need to put data in a cell before you move on. I have attached a file to show you what I am talking about
formatting.zip
Don