Using The Result Of A SELECT As A Vaule To UPDATE With

Hi AllI hope some1 with more experience of Sql Server 2000 Stored proc's can help me or point me to somewhere I can find the info cos at the moment it's doing my head in, I have spent 2 day,s so far trying to solve the following query, the problem is I do not understand the errors I am getting or why so here is the code that won't work

ALTER PROCEDURE  dbo.UpdateStock

@OrderID intASSELECT    (dbo.Products.Stock - dbo.OrderDetails.Quantity) AS NewStockFROM         dbo.OrderDetails INNER JOIN dbo.Products ON dbo.OrderDetails.ProductID = dbo.Products.ProductIDWHERE     (dbo.OrderDetails.OrderID = @OrderID)                          UPDATE    dbo.Products                           SET  dbo.Products.Stock = NewStockWhat I am trying to do is get the OrderDetails.Quantity form the  OrderDetails table the UPDATE  the Stock feild in the Products Table by deducting the  OrderDetails.Quantity  From the Products.Stock  to give me the new stock levelWell it works to an extent in that if I do this to  SET  dbo.Products.Stock = 10 it will put 10 in the Stock feild but I am unabke to get the value of  NewStock , which caluculates the correct value into the set statment I keep getting invalid colum name, I have tried any combination I can think of but I still get error of 1 sort or another.So if any1 could help as I am pulling my hair out with this 1So perhaps some1 could cast there eye over it pleaseThanks