select * , (select sum(amount_paid) from tblpayments 
          where account_no = p.account_no and date_paid <= .date_paid)
             as running_total
from tblpayments

-- Prepare sample data
USE TempDB

SET NOCOUNT ON

CREATE TABLE	dbo.Payments
		(
			Account VARCHAR(5),
			AmountPaid MONEY,
			DatePaid DATETIME,
			RunningTotal MONEY
		)

DECLARE	@Loop INT
SET @Loop = 1

WHILE @Loop <= 500000
	BEGIN
		INSERT	dbo.Payments
		SELECT	ABS(CHECKSUM(NEWID())) % 10000,
			ABS(CHECKSUM(NEWID())) % 10000,
			ABS(CHECKSUM(NEWID())) % 10000,
			0

		SET @Loop = @Loop + 1
	END

CREATE CLUSTERED INDEX IX_Payments ON dbo.Payments (Account, DatePaid)

-- Stage the data
DECLARE	@Total MONEY,
	@Account VARCHAR(5)

SELECT	@Total = 0,
	@Account = ''

UPDATE	dbo.Payments
SET	@Total = RunningTotal = CASE WHEN @Account = Account THEN @Total + AmountPaid ELSE AmountPaid END,
	@Account = Account

-- Show the expected output
SELECT		Account,
		AmountPaid,
		DatePaid,
		RunningTotal
FROM		Payments 
ORDER BY	Account,
		DatePaid

-- Clean up
DROP TABLE	dbo.Payments