In TFS 2010 we use collections, collections act as different “Servers” that the team can use in order to create separation between different groups of projects.
The build services can service one collection at a time.
In order to change the collection that the Build Services is servicing do the following:
Step 1: Launch the “Team Foundation Administration Console”
Step 2: Choose the “Team Foundation Build Configuration” menu item
You can see the collection that is being served at point 1.
Step 3: Stop the build service. (Point 2)
Sep 4: Click on the build “Properties” option
Step 5: In the properties window click the “Browse…” button.
Step 6: To choose the target collection mark it and click the “Connect” button
Step 7: Now that you have all in place start the build service.
Note: Some times it will fail to restart, just retype the Password of the account that runs the build service and try again.
At this point you will have the build service up and running, if it is the first time you specify the build services to service this collection you will need top create a controller and an agent.
Step 8: Click the “New Controller”.
Step 9: Choose a name for the controller and click “OK”.

Step 10: Wait until the controller will be ready and add a new Agent
Step 11: Choose a name for the Agent, you can also add tags and set the working folder for the agent, when done click “OK”
Step 12: See that all are in read state in order to start creating builds, you can also add more agents for this controller.
Now all is done you can go to the collection’s projects and use the build services.
Have Fun!!!