Can't see other computers!! WHY

I have setup many small networks without a hitch, ususally!

Why is it that when everything is setup correctls (all WIndowsXP PCs)
each PC can't see the other BUT internet connection sharing is
working!

The setup:

All PCs running WindowsXP

The PC with the shared internet connection has it's IP address set to
192.168.0.1.

The workgroup on each is WORKGROUP

There are NO passwords

Drives and folders have been shared on all PCs

Printer & file sharing is installed

In short, everything seems to be setup correctly BUT, any attempt to
browse the workgroup fails with and error similar to "you may not have
permission ........................."

Any ideas?

Regards

John