capture AR balance for two years

Hi.
I have a table name (customer_credit_history) which has columns CustomerID, month_invoiced, year_invoiced, Invoice_sales,  Amt_paid.  I need to capture AR balance for the lat couple years by periods and year. my biggest problem with this query is caputering the value for the previous period and adding it to the next without getting another column.
The calculation is simply (Invoice_sales -  Amt_paid).  I keep getting the in multiple columns when I need it all in one row.  
customer 12345 bought on a 1000 and paid 200 ...
then buy: 900  Paid  500:   buy for 500  Paid 500.  Buy for 2000  Paid: 2200.....
 Expected results:
Customer_id,            Year              Month            AR_Bal
12345                      2008               12                  800
12345                      2009               1                    1200
12345                      2009               3                    1200
12345                      2009               4                    1000

Thanks in advance.