Application Lock

I have a question related to implementing application locks. We have two idential apllication processes written in C# which call stored procedures. Both processes access the same table, and potentially could race to access the same row. The work involves fetching one record in a table and calling some web service to determine whether it needs to change it, and apply the change if necessary.

We want to ensure that only 1 process is "working" on a given record, i.e. if process A has access to the record then process B should let it go, and vice versa.

We're thinking of introducing a column in the table called "locked", and create a stored procedure to "lock" and "unlock" a given record.

So process A or B will do something like (pseudo code):

1- call sp to "lock" the record
2- call web service to determine what to do with it
3- make update to the record
4- call sp to "unlock" the record

For the store procedure to lock, would the following work:

------------------

UPDATE some_table
WHERE locked = false AND id = @record_id
SET locked = true

-- see if we acquired the lock
IF @@ROWCOUNT == 1

------------------

The update statement checks if the row wasn't "locked", and "locks" it only if it's not "locked".

My question is would this work? i.e. would the update statement execute atomically, i.e. test if lock is not acquired, and acquire it by flagging it as "locked"?

Thanks,
Aladin