I am in a similar position with the 8800 GTS. The issue is not just with the HP Pavillion however as I tried it with several other HDTV's at the retailer and I got the same result.
To be 100% accurate, I had initially tried the ATI 2900xt card and it is the one I tried at the retailer with my PC. The 8800GTS exhibits the exact same issues with the addition of the boot sequence mess.
I got a tip on another forum to check out the DVI Detective from Gefen which basically holds in state an EDID (config for lack of a better word as I am now to this part of the HDMI lingo). http://www.gefen.com/kvm/product.jsp?prod_id=1378
Basically you attach it to you TV and turn the TV on. It copies the EDID signal and stores it. You then attach the PC to the other end and this this device between the two regardless of turning off the TV, switching inputs etc the PC still thinks it is connected at the EDID you want and all should be good.
Been trying to order on through a local dealer and they are very responsive however. I may have to go direct. Not even sure if it will fix the problem ,but hex editors and messing with the firmware are probably a little beyond me at this point.
I am not as worried about the boot sequence as I am about the windows Vista desktop. It will not sync at 1920 x 1080p @ 60hz. I think others are saying that they have gotten this to work. Is that true? If I use the HDMI 1 with "PC" mode I get a windowed image with a 6-8 inch black border around it. If I go with the "normal" input mode I get closer to filling the screen, but still with a 1-2 inch black border.
I can get a signal with 1920 x 1080 @ 59hz and 30hz, but they each have varying degrees of overscan where the image spills over the edges of the screen border.
If I could get either 1920 x 1080p @60hz or 1920 x 540i 60z (or would that be 30hz) out of the card I would probably be fine as possible either the TV directly or routing the signal through my Anthem Statement D2 would allow for a clean signal.
In either case, my experimentations either direct of through the D2 have so far given the same disappointing results. I need more time playing around I guess.
It is very frustrating when I used to use a ATI Radeon 256mb card at a fraction of the price and it worked fine. Pixel mapped perfectly out of the box in Win XP to the HDMI 1 "PC" mode.
I keep coming back to Vista being the issue or the NVidia Drivers for Vista. It just doesn't make any sense that the card wouldn't work.
Anyway, any suggesting would be helpful. Lots of info here, and I have another thread going on the HTPC side trying to troubleshoot this.