SQL Server doesn't have a mechanism for doing pessimistic locking. You'll have to implement something yourself. A common method is to set a flag on the row or have a seperate table that contains the ID of the rows that should be considered locked.
Now there are ways to do what you want but they're not recommended. You can set your transaction isolation level to REPEATABLE READ and do a SELECT on the row first. You could also use a lock hint and get the same effect. The downside is that you'll have to start a transaction and hold the transaction through the life of the edit - not a good idea IMO. You can affect the concurrency of your application greatly.
-Mike