1) First check you have the analysis toolpak installed as part of excel (just in case :-)
2) There are some hints and tips on using excels financial functions at http://www.sytsma.com/cism700/excelfinfcns.html
3) http://chadrowe.com/excel/examples/xnpv.htm shows some examples of using excel's fin. functions
4) http://finance.wharton.upenn.edu/~benninga/mit/additional.html has an example spreadsheet,
 and documentation in pdf form which may be helpful (I attach the relevant download - introfin.zip)
4a) Winzip - to unzip my attachment if you don't have it already - is available from http://www.winzip.com/upgradex.htm
4b) Pdf i.e. acrobat reader, is available from adobe - http://www.adobe.com/products/acrobat/readstep.html - if you don't already have it.
5) If all this still doesn't solve the problem (i.e. trying to use Excel's built-in function), but you have a formula already
 which you know works, please send it for translation to excel format
 - apologies if you wanted to jump straight
 to this step, I was kind of working on the assumption of trying the easy things first :-)
6) If you _don't_ have a suitable algorithm, there is one available at http://finance.bi.no/~bernt/gcc_prog/algoritms/algoritms/node5.html , though
 it's in C, so you may want to get me or Marcus to translate!