Does anybody know how I can reinstall a codec that came with XP? Because now since I accidentally uninstalled the wrong codec I can't play my MP3's. Any help would be appreciated.

Check the mp3 file with "G-Spot" (a tool for codec information) what codec is missing.

When you know what codec you need, just install it.

The way codecs work, if you have multiple codecs contending to play back the piece of content and the one that's being used is broken - you would likely want to remove that "primary" codec and let Windows use the one it normally would be using.

There should be no need to reinstall the default codec here - you should just need to remove the bad one. =\