When you get the warning, you should see a little box appear that tells you which cell has the circular reference. Basically it means that the from cell A is reading from Cell B, but Cell B is reading from Cell A, so the formula is going round and round
Cell A1 formula = B1+C1
Cell B1 formula = A1+C1
Once you know what cell has the problem then you can change the formula.
Alternatively, if you need the circular reference to work you can set the maximum number of iterations for cell calculation (up to '03: Tools ->Options ->Calculation ->enable iteration, '07: Office button -> Excel Options -> Formulas -> enable iteration), killing the message that way.