if you want a hardware solution you can get a 2 way dvi hdtv splitter and amplifier which you plug one dvi cable into and it splits it into 2, one for you monitor and one for you hdtv.
they are pretty costly and your screen and hdtv would have to use the same res.
so i would advise using the software equivilent (if you graphics card has 2 outputs) download ultramon and use a dual monitor setup,
in ultramon you can setup display profiles, which are interchangable with 2 clicks of your mouse. e.g.
profile 1 = 1680x1050 res on monitor 1, monitor 2 off
profile 2 = 1366x768 res on moniror 2, monitor 1 off
hope this helps,
StevenG