You might want to check out the IPMT function. From the XL help file:

IPMT(rate,per,nper,pv,fv,type)

Rate is the interest rate per period.

Per is the period for which you want to find the interest and must be in

the range 1 to nper.

Nper is the total number of payment periods in an annuity.

Pv is the present value, or the lump-sum amount that a series of future

payments is worth right now.

Fv is the future value, or a cash balance you want to attain after the

last payment is made. If fv is omitted, it is assumed to be 0 (the future

value of a loan, for example, is 0).

Type is the number 0 or 1 and indicates when payments are due. If type is

omitted, it is assumed to be 0.

Set type equal to If payments are due

0 At the end of the period

1 At the beginning of the period