Suppose cells A1:C5 look like this:

interest_rate: 0.05
date payment balance
1/1/2007 10000.00
1/28/2007 100 9936.16
2/20/2007 105 9861.75

The formula for cell C4 is =C3*(1+C$1)^(INT(A4-A3)/365)-B4
The formula for cell C5 is =C4*(1+C$1)^(INT(A5-A4)/365)-B5

Copy the formula for as many dates and payments as needed.

Hth,
Merjet