The short answer: You can't.

The somewhat longer answer is that at least as far as Windows services go, those are system wide and are the same for all users. Most user space programs, like those you install yourself, you can prevent from running, though some of them do like to lodge themselves in a part of the registry used to tell the OS what programs to load when any user logs in, so if you disable the program there, then you'd need t go in and manually set it to load with specific user accounts if you wanted it to load every time otherwise.

Basically the user accounts on Windows are rather superficial things, allowing for different wallpaper and start menus, but little else when you really get down to it. They don't offer the kind of fine grained tuning you want. You'd probably be better served with a video player like VLC which uses a custom developed collection of video codecs that are extremely optimized for decoding, making use of CPU features like SSE and MMX whenever possible. Even more than that, there is an option in VLC to boost the priority of the decoding process during playback, so that the OS makes sure that process gets CPU time when it requests it. VLC also supports DXVA2 playback for the formats supported by your video card. That offloads all the video decoding work to the GPU instead of the CPU, since the GPU is quite literally designed for that sort of thing it is quite a bit more efficient about it. Those things together will do far more than what you were planning.