Here is a short tutorial on switching a SQL Server 2005 instance to single user mode:

1. Stop the service:

  Using a command prompt type: net stop MSSQL$<your_instanceName>

2. Start the service in single user mode:

  Using a command prompt type: net start MSSQL$<your_instanceName> /m

3. Do all of your your stuff using the only available connection...

4. Stop the service again

5. Start the service again

Using a command prompt type: net start MSSQL$<your_instanceName> but this time without the /m parameter

Note: To get all currently opened connections you could execute the following query:

SELECT * FROM sys.dm_exec_Connections