Need help (problem downloading files, and watching streaming videos)

Hello, I wasnt sure the best place to post this, as it could pretain to both windows XP or networking problem. I am encountering a few issues downloading things, and watching streaming videos. For example when I download something (game demos, softwear, and basically anything of internet explorer) it will download the full thing and when it finishes it says the file i've download is corrupt, and does not install. I've cleared all my temp files (that I know how to clear) and tryed defraging and error checking, i've done virus checks. Also I seem to be for the most part incapable of watching streaming videos, such as youtube videos, and other streaming players. The videos start playing but sometime before the end will just stop and not continue to play, despite the fact that the video is fully loaded. This has been a long standing problem, and I seem unable to solve it on my own. So if anyone can help me with this it would be of great appreciation as it causes me a lot of agrivation.