Can't help you with the function I'm afraid, but if you have MS Works there is a ready made Loan Analysis spreadsheet. I just tried it based on your figures. All the formula's are IF Statements rather than Functions though.
I could email you the IF statements but you would need to change the cell references to your own?
The results I got were (if it is any good to you):
Monthly payments 415.76
Total number of payments 36
Yearly principal + interest 4,989.08
Principal amount 13,872.00
Finance charges 1,095.23
Total cost 14,967.23