As Niek has asked, it's probably a good thing to see the formulas you're using.
You CAN turn off the checks for circular references via:
Tools | Options | [Calculation] tab and check the Iterations option
This is rather dangerous thing to do because it will completely hide any
unintentional circular references you may either already have set up or may
set up in the future.
Quite often there is a solution that can be worked out that doesn't involve
a circular reference, and that is definitely more desirable - but to begin to
figure out such a solution, the formulas in use would have to be seen.
