From what I know is that a HD(High Definition) movie format
It's not a "format" really- it's just a standard resolution. 1920x1080 is HD; standard NTSC is 320x200 (if I remember correctly), etc.
From what I've read, Win7 is better at handling HD video. Is there any truth to that?
None at all.
AMD 2.4ghz dual core, 3gb ram, Nvidia 8400gs 256mb
You beat the bare minimum requirements recommended for HD video playback; except that the video card should have a core clock of at least 600Mhz; the 8400GS gas a core clock of 567; not a huge difference, but recall that those were the absolute bare minimum requirements; consider that the "minimum" requirements almost never give good results.
It's <possible> to get it to work with lower specifications, because the popular codecs like FFDShow, Klite pack, CCCP, and the one built in to VLC do not make the optimal usage of the Hardware; generally speaking, they almost always do a large amount of the decoding task on the CPU, which is why the minimum requirement needs a dual-core or a fast single-core.
The only decoders I've been able to use that shirk that requirement are those included with or written for the video card (for example, the ATI Media player program that is included with many ATI cards), as well as the Nvidia decoder software, which works hand-in-hand with a supported GPU. Additionally there are "optimized" codecs that work in software mode, such as CoreAVC. The problem is of course that all of these are commercial (except for the ATI media player which comes with ATI cards, but trust me, I make it sound better then it really is).
Basically, the recorded resolution of the camera is simply too much for the computer to handle without a lot of optimization; generally speaking it's got to push about 2,073,600 pixels for every single frame, each pixel has at least 24 bits of colour information, so that's almost 6.2MB of raw information for a single frame, at a standard 24fps that's over 149MB every second! If the camcorder records the data "raw" then your hard disk will almost certainly be a bottleneck; most HD's can only exceed 150Mbps or so in bursts. If it compresses it, then your computer has to read the compresses data, decode it and expand it into the full 149MB, and then send that all along to the video subsystem. sometimes the codecs themselves offload the actual coding/decoding to the video card if possible, so that can speed it up, but it's still a lot of information to process; the camcorder->TV connection uses the camcorder, which has a special hardware configuration specifically designed for video playback/recording.