I know both have problems, but from what i gather, vista more than xp.
In xp the fix is a simple... control panel settings directly to realtek with the latest Realtek drivers. Sometimes it's good to remove old drivers before installing new ones to ensure no further conflicts due to old driver remnants, I know I had problems with a video driver because of that, so the extra precaution couldn't hurt if the problem persists after basic tinkering with settings.
I don't use vista though, I have way too many media progies working correctly in xp to invest in new bugs, lol.