Having issues with HD video outputting onto my computer?

So i have a Nikon P520 and it records up to full 1080HD video. This cameras like brand new, it was a gift for christmas. The SD card i use is a Sandisk Ultra 8GB. On my camera the video looks crisp and full HD. But once uploaded to my computer (its also a newer computer, less than a year old) it lessens the quality. Once put into a video editor, Windows Movie Maker, the quality decreases even more. By the time i'm finished editing the final product is 480p, but i mean its a high quality 480. What could be decreasing the quality?

Windows movie maker will definitly be a big part of the problem. As it uses.wmv only as a format. And if I'm correct and using dslr camera usual format.mp4 there's an assumption that it shouldn't work with windows movie maker unless a new version of it reformats the codec, so the possibility would be a recompression or settings on your dslr, the small viewfinder may seem to be doing 1080p but it will be your settings which will confirm this, pending settings being at 1080p, the most likely reason will be windows movie maker. Loathed by video professionals since 1995

Just about everything - photos and videos - looks good when previewed on the camera. Once you get it enlarged on your computer screen more of the details show and it never looks as good as it did in the camera:-)

Uploading to your computer can't decrease the quality or size of a video and opening the file in Movie Maker isn't going to decrease the size/resolution from 1080p to 480p. The file is just moved from the camera onto the hard drive and Movie Maker just links to the video where it is. The file isn't even moved. There's nothing that would change the video in any way. You would have to save to 480p for an edited video to be 480p.

You don't say that you're converting the video clips to use in Movie Maker, so I'll assume the file comes out of your camera, onto your hard drive and, from the HD, is used by Movie Maker - with no converting to another file type.

Go to My Video or wherever you save your video files to. Right click>Properties on one of the videos you've uploaded from your camera. Click on the Details tab at the top of the Properties dialog. Write down the following information:
Width
Height
Bit Rate
Frame Rate

scroll down to the Audio sections and write down the numbers that go in the blanks below:
____Kbps
____KHz
____Mono or Stereo

Open a clip in Movie Maker, click on Save Movie and, at the very bottom, click on "Create Custom Setting…".
Setting = <NEW> (don't need to change)
Name = 1080p HD
Width = 1920
Height = 1080
Bitrate - a number up to 135,000
Frame Rate - a number up to 60
And, in Audio, use the dropdown to select your video clips' audio information.

Increasing the Bitrate is going to improve the quality. 480p is not "quality". It's resolution/size. Increasing the bitrate to something closer to that of the clip is going to really increase the size of the video file. Most of us like to find a "happy medium" between the bit rate and the file size. If you're uploading videos to YouTube, you don't want to wait 8 hours for a 35 minute video to upload:-)

(There are 1080p choices in the Save Movie dropdown>Common settings>High Definition Display. They save in 1080p, not 480p.)