Why do videos I take on my DSLR play slow on my computer when I convert them?

Why do videos I take on my DSLR play slow on my computer when I convert them?

I have a Nikon D7000 (16.2MP 1080P) and I use it for pictures and video, I used to use NEFS but found they weren't at all improving the quality of my pictures.
My video I take on the camera and when I upload it to my computer through a SanDiskHC (8gb) it does come through and I copy them over to my computer which there's no problem with. The Problem occurs when I go to watch those video on either iTunes or Windows' Media player, and find that they come through and play in really bad quality, the footage plays really slow as in like roughly 10 frames per second which is very poor, as well as the sound quality being rubbish.

I want to know why this is and how I can prevent this from happening so I can take good quality films on my DSLR and be able to watch them in the same good quality on my computer.

Thank you in advance.

Added (1). @Jim, Hi thanks for your advice, I'm currently using an Alienware M15x 2.8GhZ I7 with an Nvidia Graphics card of 1.5gb and RAM at 6GB, So this is surprising, from what you're saying, I'll try my dad's computer and see if the problem persists. Cheers for your help!

What software do you use?

I also have an i7, 2, 2ghz and an Nvidia video card with Cuda support.
BUT I also use software that supports Cuda. And that makes a hell of a difference.

The processors we use are made for running well everything so they might have a few optimizations but they are not "stream processors" a stream processor is bad at generic tasks BUT. Give it a simple job to do and it is FAST.

See a stream processor as a person who works at Mc's only making burgers. Thats it no fries no shakes no drinks no helping customers. Just making burgers. They are fast it

See the person preparing the order packing everything and setting it on the tray as the generic processor.

So to unlock the power of your laptop/pc you need software to use your video card. Cuda is quick darn quick.very quick. It eats video data for breakfast!

So make sure the software works with your video card. And yes. HD video means more data. And processors made for generic tasks are able to do allot of different things. But they are limited in performance for specific tasks.

That and even then. There's a certain time it takes. Converting a 10 Minute video can take 3 minutes here.yup 3 minutes.sometimes 5 depending on the codec used. I shot some videos at Max Coded my A65 could handle and recoding that to something youtube would like. It was a 4 minute video. 720p MP4 output. 3-4 minutes.

But if I keep the resolution the same it doesn't needs to downsample and it goes further.

Hey big files, lotsa data. Its the price we way. Make sure that when encoding video you run your laptop/pc in full performance mode.