There are a lot of unknowns in the network configuration, so this is a guess.
It could be that your ISP's router has wireless enabled and you're connecting to it rather than your Belkin.
The first thing I'd do is ensure that you are connecting to your Belkin router and not the ISP's wireless. If you are, see if you can disable the ISP's wireless radio to avoid others piggybacking the connection. It's also possible that you can see other customers on the ISP's network if you're connected directly to their router and not your Belkin.
I don't know about Win7, but suspect that it has a default workgroup name, and the other PCs you see had the same default. In that case, you'd see them, but the Win7 firewall settings would prevent connection to them.
Also, I'd call the ISP to get any remaining questions answered;
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