Welcome to Windows Seven Forums, Wayne.
You need to forget about the TV issue for now and concentrate for now on why you can't get to the desktop.
If the store has been able to hook it up to a different monitor, that suggests to me that your monitor may be the problem.
Check that the cable between the monitor and computer is secure and that the monitor is switched on (don't laugh, I've seen it happen!
Is there more than one DVI socket on the graphics card? If so, have you tried both of them?
Do you have another monitor you can try? Did your monitor come with the computer? If so, you need to pressure your reseller for a replacement.