OK, this is a strange one, so excuse pardon me if I include too much detail.

Windows Vista Ultimate
HDMI Video out to a Plasma TV
Optical Audio out to a receiver
Snapstream Firefly Remote (Detected as X10 remote, newest Firefly Software Installed)
Belkin Media RF Keyboard
Abit Fatality MB with ATI (DirectX 9) HDMI output
Power Settings: High Performance. 
  Turn off the display: 10 minutes
  Put the computer to sleep: never

When I first start Media Center everything is fine.  I can navigate with the remote all I want.  Once I play the first video, it all changes.  Whenever I return to a menu from a video, whether it finishes, or I just hit the back button and let the video continue, the TV immediately loses the video signal.  Audio is fine, so I know everything is playing ahead just fine, but the TV pops up the "No Signal" message.  If I hit any key on the wireless keyboard it wakes up.  Key presses on the remote do not return the video, but are being received, as I can navigate and play another video or audio file (blindly) and video will return as soon as the media starts playing.

The only other issue I have noticed is that sometimes on a PC restart the remote will stop working.  The only thing I can do to fix it is uninstall the Firefly software, reboot, and reinstall.  I simple reboot does not help.

Any help would be greatly appreciated. I'm sorry if I missed a thread with the fix in it feel free to point and laught at me, as long as you point me towards the thread as wel.

Thanks.

i don't have the firefly but i have a X10 based remote (UR86EL).

What i did find is that most software implementations of X10, most propably the firefly software as well, does not use the keyboard hooks directly. screen savers and power saver options mostly look as these hooks. that is why the keyboard works fine.

the only real solution for the screen that goes blank is to disable that power saving option, by opening control panel, then power options and selecting high performance.

as for your issue with the CM19 or CM21E USB receiver that stops working, i've had that same issue but luckily a cold boot resloves my USB issues.

I'm actually having the same problem on my system except I have it hooked to a projector and am using an ATI remote wonder. Disabling all the power saving features didn't seem to help me out at all I just have to get up every time and push alt+f4 and esc a few times to get it to come back whenever it does it.
The same issue seems to affect windows media player also. As soon as a video is done playing the signal drops.

I would say you have the same problem described here http://thegreenbutton.com/forums/thread/195205.aspx  and to-date there is no solution other than to set screen timeout to "never"