SQL-Server: Error - Exclusive access could not be obtained because the database is in use

I am actually trying to make a script (in Sql Server 2008) to restore one database from one backup file. I made the following code and I am getting an error -

Msg 3101, Level 16, State 1, Line 3
Exclusive access could not be obtained because 
the database is in use.
Msg 3013, Level 16, State 1, Line 3
RESTORE DATABASE is terminating abnormally.

How do I fix this problem ?

IF DB_ID('AdventureWorksDW') IS NOT NULL 
BEGIN 
RESTORE DATABASE [AdventureWorksDW] 
FILE = N'AdventureWorksDW_Data' 
FROM  
DISK = N'C:\Program Files\Microsoft SQL Server\
MSSQL10_50.SS2008\MSSQL\Backup\AdventureWorksDW.bak' 
WITH  FILE = 1, 
MOVE N'AdventureWorksDW_Data' 
TO N'C:\Program Files\Microsoft SQL Server\
MSSQL10_50.SS2008\MSSQL\DATA\AdventureWorksDW.mdf', 
MOVE N'AdventureWorksDW_Log'  
TO N'C:\Program Files\Microsoft SQL Server\
MSSQL10_50.SS2008\MSSQL\DATA\AdventureWorksDW_0.LDF', 
NOUNLOAD,  STATS = 10 
END

Solution

I'll assume that if you're restoring a db, you don't care about any existing transactions on that db. Right? If so, this should work for you:

USE master
GO

ALTER DATABASE AdventureWorksDW
SET SINGLE_USER
--This rolls back all uncommitted transactions in the db.
WITH ROLLBACK IMMEDIATE
GO

RESTORE DATABASE AdventureWorksDW
FROM ...
...
GO

Now, one additional item to be aware. After you set the db into single user mode, someone else may attempt to connect to the db. If they succeed, you won't be able to proceed with your restore. It's a race! My suggestion is to run all three statements at once.