Bad HD (720p) video play back

[orange]

get kmplayer for windows (not the linux one with same name)
it has its own codecs (like mplayer) and its highly configurable. you can use the 'low speed' preset.
it doesn't even need installing/messing_up_registry, just unpack it somewhere

[orange]

well the only thing left is mplayer... if you aren't bored by now
it also has its own codecs and doesn't need installation.
however it lacks usable GUI (there are various, but I couldn't find a decent one)
it seems to me like the fastest possible player out there

I'd also suggest defragmenting the HDD partition that holds the movie and killing unneeded processes/services