cumulative sum in sqlServer

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SQL-Server-2005

SQL-Server

I want Cumulative Sum on SalesAmount Column from below given table structure.

Ex:

ID       SalesCode              SalesAmount
1            1                       100
2            1                       500
3            1                       1000  
4            2                       15000
5            2                       10000
6            2                       25000  
7            2                       35000

I Want to Result like this:

ID       SalesCode              SalesAmount             SumAmount
1            1                       100                   100
2            1                       500                   600
3            1                       1000                  1600 
4            2                       15000                 15000
5            2                       10000                 25000
6            2                       25000                 50000
7            2                       35000                 85000

I am using below given query, but its very slow, as I have more than 10 lakh record in this table.

Kindly suggest me the better way forward to get solution in min. time.

the Query what i m using is :

Select S.Id, S.SalesCode, S.SalesAmount ,
Isnull((Select Sum(SS.SalesAmount) From tblSales SS Where SS.SalesCode = S.SalesCode And SS.ID <= S.ID), 0) As TotalSales
From tblSales As S