I had same with ac97 sound on a vista desktop - in the end I dumped a soundcard on it - one way to crack the problem, but I guess you don't have the option in a lappy. If it shows up in dev mgr but not in sounds speech & audio devices then I guess it's either dead or possibly bad drivers/software.
I would maybe uninstall it again, clean the registry & then download the latest drivers/apps from the Realtek site - I've known bad drivers from vendors before. One final possibility id if it shows up ok in dev mgr, it may have become physically disconnected from the mobo through thermal shock or having been droppped. Perhaps opening up the case and looking will help?