So, let's create arbitrary payments x and arbitrary time of payments t and generate the data for a Cashflow object. For concreteness, I'm letting there be 10 times at which payments are made.
cfdata= Transpose[{Array[x, {10}], Array[t, {10}]}]
We can now calculate the present value of the cashflow as follows, where r is some interest rate or the result of an EffectiveInterest computation.
TimeValue[Cashflow[cfdata],r,0]
To compute the present value of the remaining payment stream as one moves through time, one could do this:
Table[TimeValue[Cashflow[Drop[cfdata, i]], r, 0], {i, 0, 10}]