AVI files quality reduces after edit and upload

I recently purchased a Nikon D5000. I'm using the camera to make videos and upload them to youtube.
When I first upload my video onto my computer the quality looks great but after I edit the video into windows movie maker and upload it onto youtube the quality reduces greatly!

Why is this and what can I do to fix it? Do I need to get a different editing program or save my files differently? Please give me any recommendations you can.

WMM is an okay program but you're correct, it does reduce quality.

Buy a real editor and that won't happen. I use Video Pad by NCH software… An excellent editor for about $100 retail. Also I have Creator by Roxio - another excellent editor… Also about $100 at retail.

I shoot video with my Canon t1i and here's an example of what I'm getting through Video Pad. This format is mov not avi but the principals are the same.

Did you trying using a different hosting service? Like www.4everload.com

Yeah. The older versions of WMM can't save in HD (although I believe WMM Live can). But all of them, even WMM Live, are very limited in what they can do. Would instead recommend several new freeware programs that are fully HD capable, such as Freemake, FileLab and VSDC. See the following link, think it will be of interest:
http://ezvideoedit.blogspot.com/2012/10/joining-merging-splitting-cutting-and.html

You may try AVS Video Editor, it's a good video editing program for windows.