OK so I have downloadedthis xvid codec thing so I can watch my movie online (like the site told me to) and I DO NOT know how to use it... so can please walk me through it for someone who is like overally computer illterate... just tell me how to use the xvid coec to watch my movie. Like step by step... and I have an acer computer with widows xp?

If you install a codec it sort of becomes part of your operating system and is used automatically by your media players installed.
If, like you said, you are trying to play if from you internet browser then try updating your browser or try a different one.

If that doesn't work then download and install "XP Codec Pack".

Download XP Codec Pack 2.5.1 – XP Codec Pack is one of the most complete codec packs