In short it sounds like a settings issue. Nvidia has their own control panel so be sure to use that.
I have an nvidia 9800gt that i am trying to connect to a samsung led tv via dvi to hdmi cable. in my nvidia control panel it does NOT let me select what type of connector i am using, and has vga listed for both my tv, and the monitor i am using currently (an old lcd using a dvi to vga adapter). it may be as simple as finding drivers that lets me change this, but i have used the most recent, version 266.58 and ones from 2010 (when this was my main computer) and none would allow that.
the tv will work fine during booting, even loading windows, however as soon as it should go to the user account screen, the tv will lose signal. it also works fully when booted in safe mode, although there i believe it will not let me use full 1366x720 resolution.
im using w7 64 ult, 4gb ram, e4500 c2d
apologies if i left out any info, i'll be happy to add more. thanks