If it were a pinched cable, then you wouldn't be getting ANY video, it wouldn't be just that you lose video once Windows starts loading.
This sounds like a software issue, such as the internal display being set to a resolution not supported by the panel. HOWEVER, when I do stop and think about it, I ran into something like this once in my hardware monkey days, and it was ultimately a bad GPU. This was one of the infamous GeForce 8600s though, but the symptoms were pretty similar. Things worked fine until the OS loaded.
That being said, if you plan to send it in, then the more evidence you can provide the better. If you can make a quick video with a cell phone or something and then burn that to a CD and include it with the system, do that. I hate to have to say this next part, but make sure it's in a format like WMV so that the repair tech doesn't need to go hunting for codecs. You basically have to remember that repair techs are often very busy, and so they can't always spend as much time as they might like on any given unit. The more evidence you can provide to back up your assertion that the problem is X, the better. They will have to duplicate it themselves, but if you can provide a compelling argument, it shouldn't be hard for them to do.