Transact SQL :: Subtract Two Columns With Windows Function

How can i subtract two columns:

I have table:
       
ID COL 1 COL 2
-------------------------
1 200.00 70.00
2 200.00 30.00
3 200.00 90.00
4 200.00 110.00Col1 - COL2

But to continue for each row and value is reduced by the previously value.

My output should be like as:

ID COL 1 COL 2 [COL3 AS RESULT]
-------------------------
1 200.00 70.00 130
2 200.00 30.00 100
3 200.00 90.00 10
4 200.00 110.00 -100