Imagine you want to change the format of A2 depending on the value in
B2. Select cell A2 and click on Format | Conditional Formatting, and
in the pop-up you should choose Formula Is rather than cell Value Is
in the first box. Then in the next box you can enter this formula:
=A2>B2
Click on the Format button and then choose the Patterns tab (for
background colour) and choose a colour. Click OK twice to exit the
dialogue box.
Now if A2 is larger than B2 the background colour will change. If A2
is not larger than B2 then the background colour will revert to
normal.
That's how I used to do it in Excel 2003, but I'm not sure how to get there
in Excel 2007. I can't find the pop up to change from "formula is" to "cell
value" is.