Sounds like a big-ish project? You choice of excel makes me think you're using it at a small corporate (not large and also not research). To get it "right" (and validated?) you'll probably end up having to implement a lot of details like daycount conventions, bootstrapping etc etc. It this is the case then you might be better of to invest time into learning some framework that has all these details already setup for you. I've heard that quant-lib is used a lot, and there is an Excel addin:
http://quantlib.org/quantlibxl/