Restore SQL Server 2000 Database in use
Running sp_who2 will show you who is connected to the database.

The safest way to disconnect users is to ask them to disconnect themselves. If that is not practical, you can take the database offline or set it to SINGLE_USER mode to force the other connections out before performing your restore.

See this for more information and examples:

ALTER DATABASE [Test4] SET SINGLE_USER WITH ROLLBACK IMMEDIATE  

RESTORE DATABASE [Test4] 
FROM DISK = 'c:\test4.BAK' 
WITH MOVE 'Test4_Data' TO 'c:\data\Test4.mdf', 
MOVE 'Test4_Log' TO 'c:\data\Test4_log.ldf'