I am not expert on this - but perhaps this is why:
"These days almost every home has a computer network. But if one of the PCs in your home uses Windows XP, you need to change the WORKGROUP name in order to let your Windows 7 PC recognize the Windows XP PCs on the home network.
In order for PCs to find each other, they all have to have to be in the same workgroup. For some reason, Microsoft decided to call Windows XP Home workgroups MSHOME. On the other hand, Windows XP Professional, Windows Vista, and Windows 7 PCs all use WORKGROUP as their workgroup name.
Consequently, unless you change the workgroup name on the Windows XP Home PC, the other PCs can find it and it can’t find them." click here
(I think that means XP can see W7 but W7 can't see XP)