Sooner or later you will find Excel complaining about Circular References. This is often caused by a typing error (I do it all the time!) so I'll take a moment to tell you what it means and how to fix it.
The example below shows the problem in its simplest form. Ive put the formula =C3 in D3 and =D3 in 'C3'.
The value in C3 depends on the value in D3 and the value in D3 depends on the value in C3, there is no solution (in this case any value would satisfy the conditions).
Circular references can be easy to spot, or it can take hours to puzzle out the problem. You might have to trace back through several links in the chain until you find the loop. It may be an obvious mistake, or it may be quite a subtle problem with your logic.
This example shows the chain of links that involves four cells. D7=D3; D3=C3, C3=C7; C7=D7:
You can follow the blue 'precedence' or 'dependence' arrows to help figure out the problem. Otherwise check out the Help system to find more powerful diagnostic tools.