Sounds like the resolution or refresh rate settings got changed for your HDTV after the new driver was installed.
Try these steps:
1. Open up the Intel graphics driver's user interface. For help on how to do this, see: http://www.intel.com/support/graphics/sb/CS-022130.htm
2. Select Display Settings tab.
3. Adjust the resolution and/or refresh rate to ones supported by your HDTV. You may need to check the HDTV's manual for what's supported.
4. Click Apply or OK.
5. Check if you can switch to the HDTV display without the error. You can do this from the driver UI's Display Devices tab.
If that does not help, do you still see the problem if you're using the driver from Dell's web site for your system? You may need to use the driver from Dell rather than Intel's generic one. Some computer manufacturers customize the Intel driver so that it works well with their systems.
I have a similar problem on my Asus Essentio. I am trying to use my TV as my primary display. In order to get it setup I hooked up a computer monitor also. I got the resolution set for my TV (720p@60) and I disabled the computer monitor in the graphics driver and everything looks good until I unplug the computer monitor and then I lose signal on the TV. Even if the Monitor is turned off and unplugged I still cannot disconnect it from the Asus without lossing signal to the TV.