2 ideas.
1 - is media share enabled on pc? if so have you added all the directories you want to share with xbox to the windows media player library?
2 - use a third party dlna server software. Id recomend tvercity
http://tversity.com/ install on your pctell it what folders you wish to share then xbox and ps3 can connect to the tversity server no problem. check their website for more info if needed. another advantage itll play media formats not normally supported by xbox and ps3.