Why does the video quality look bad when uploaded to the computer? [iPhone 4]?

When I upload my video to youtube in HD directly from the iphone, I go on the computer to watch it on youtube and the quality is perfectly normal. But when I copy the file from my iphone and onto my computer the video looks grainy? I wan't to put it into after effects and do some editing but how can I go about KEEPING the quality of the video? I don't get why it gets all grainy when I put it onto my computer vs. It being perfect when I upload it directly from my ipod to youtube?

Because your computer is not set up for HD videos. HD video requires the computer to have a Newer, multi processor chip, a 500 MB or bigger Graphics card and a sound card. Both of these card are mounted in expansion ports on the motherboard of your computer, Laptops cannot be upgraded this way. Also you need to running Direct X® 9 or better in your Computer for it to play, edit or work with HD video