After I trim a 12 min video to a 45 sec clip (using the program's trimming tool), no matter how I save it ( "Recommended for this Project," "YouTube," "High Definition"), the image becomes blurred in certain sections (ie, it flickers, looks sluggish/pixelated etc.)/the quality of the picture in general is off. I've recently deleted over 3G of data from my computer, and I've done this (trimmed longer videos into clips) dozens of times in the past, and, though it has happened once or twice, I always thought it was nothing; now, however, every clip I try to make is like this. What can I do?

Note: if I watch the trimmed video in MovieMaker, it appears fine; it is only when I try to watch it using Windows Media Player or something similar/upload it to YouTube that the problem occurs.

Thank you!