problem adding W7 kaaotop to network

I have an existing wired/wireless home network that has three XP
desktops. I got a Windows 7 laptop and I'm trying to add that to the
network, via wireless. The new laptop connects fine and it sees the
rest of the computers. The XP computers don't quite see it. On the
new laptop, I told it to share, etc.

On the XP desktops, under Network Places/Entire network/MS windows
network, there is
1. MShome - the old network name - it shows the old computers
2. workgroup - this is new. It shows the name of the new laptop, but
there is no "+" beside it, and the other computers can't open it.

How can I get the old XP desktops to see the new W7 laptop on the
network?
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