Conditional Insert in SQL Server 2000?

I need to do a conditional insert, and I am afraid that if I do it in 2 steps it will create bugs...
I have a table called customers and a table called locks
customers has 3 fields CustomerID, FirstName, and LastName
locks has fields LockID, CustomerID, UserID, SessionID, and TimeStamp
I need to check if there are any records in Locks for a CustomerID and if there are none, I need to insert One.
This is going to tell my application that this Customer record is locked so no one else can edit it, until the Lock record is deleted.
I just don't want to make 2 trips, One to check if the Lock exists, then One to insert the Lock, since this could allow 2 locks to be created if the timing is correct.
Any ideas or comments?This code from myPreventing Duplicate Record Insertion on Page Refresh article should do what you need (note the use of BEGIN TRANSACTION, EXISTS, and WITH (UPDLOCK

Listing 4 – Stored Procedure spAddEmployee_UsingExists

CREATEPROCEDURE spAddEmployee_UsingExists
(
      @.FirstNamevarchar(50),
      @.LastNamevarchar(50)
)
AS
DECLARE @.Resultint
BEGINTRANSACTION
IFEXISTS
(
     SELECT
           NULL
     FROM
            EmployeesWITH (UPDLOCK)
     WHERE
            FirstName = @.FirstNameAND
            LastName = @.LastName
)
     BEGIN
           SELECT @.Result = -1
     END
ELSE
     BEGIN
           INSERTINTO
                  Employees
            (
                  FirstName,
                  LastName
            )
           VALUES
            (
                  @.FirstName,
                  @.LastName
            )
           SELECT @.Result =@.@.ERROR
     END
IF @.Result <> 0
     BEGIN
           ROLLBACK
     END
ELSE
     BEGIN
           COMMIT
     END
RETURN @.Result