I've just upgraded to win7.
On my computer I can go to Start button click on connect to a projector you should get 4 options.
If you click on duplicate it will show the same on the TV as on the Monitor.
To revert back to the Monitor I have to click on Projector only don't know why could be its classing the TV as the main monitor.
Note: you may have to connect the HDMI cable to computer/TV before switching computer on.
If you do not have the option above and its a NVADIA video card right click on desktop background and select the NVADIA control panel, select multiple displays. With any luck the TV should show.
If its an ATI card it may be similar.