Recently I've built two PCs (with help from Tom's forums!) upgraded to Windows7 then homegrouped 3 computers. I added an external harddrive shared that with the network and then started storing pics/music and videos on that harddrive. I was playing some music and my wife tried to do the same at the same time. Lo and behold she couldn't access it at the same time as I!
So, what do I need to add to make the files accessible to multiple users at once?