how do you use it with window media player?

Download the latest version onto your computer and Windows should find it and it should work...wheres those experts when you need em?

What codec do you want?

windows media player should download it automatically, if it doesnt work and it error, just dont use it anymore, get winamp 5 it rules, divx player works great for avi'* also

the one to work with XP or the one you reccommend

There isn't just one WinXP codec. Every file type has a "codec" necessary to run it (some, like AVIs have several). WMP will find the one'* it can without violating license agreements (at least not blatantly), but there will be some that windows just will not play internally. You'll need the program written for that type of file.