How To Get Sum Of Previous Balance

I am grouping my reports monthly date wise. And in each month i need to get the previous sum of payments with condtions.

my table is like this:

Table Name : voucher
------------------------------------------------------------------
SlNo |  Date      | Amount | Account |PaymentOrReceipt
------------------------------------------------------------------
1       14/1/2003   100      Cash     Receipt
2       14/2/2003   100      Cash     Receipt
3       16/2/2003   10       Cash     Payment
4       18/2/2003   50       Other    Payment
5       1/3/2003    400      Cash     Receipt
6       15/3/2003   20       Cash     Payment
--------------------------------------------------------------------


Result Should be Like This (Group By Monthly)
---------------------------------------------
Month : 03/2003
CurentBalance = 380
PreviousBalance = 190 //Not 100 Cos SlNo 3 Is Not Cash Account
Total= 570
---------------------------------------------
Month : 02/2003
CurentBalance = 90
PreviousBalance = 100
Total= 190
---------------------------------------------
Month : 01/2003
CurentBalance = 100
PreviousBalance = 0
Total= 100
---------------------------------------------

I can find out everything except find out previous balance.

My Questions:
1. How to find previous Balance?
2. How to create a function using sum() function with where condtions.
3. Example Of Crystal reports (SQL Expression Field).

I am using Crystal Report 9.