Why is Windows Vista unable to see other computers and others can't see Vista machines ?

Under Vista, by default any network you connect to is considered a public (insecure) network. This may prevent discovery and file sharing. To allow Vista to access other computers on your network and vice versa, follow these steps:

1. Click Start, choose Control Panel.
2. Choose Network and Internet > Network and Sharing Center.
3. Click the "Customize" link to the right of the network heading.
4. Set the network to "Private", click Next, and then Close.
5. Look under "Sharing and Discovery" heading, make sure "Network Sharing" and "File Sharing" is turned on.