After upgrading my MacBook Pro from Tiger (10.4) to Leopard (10.5.8), I was no longer able to see the other Windows machines on my network. What appeared to work is to select Sharing from the System Preferences, clicking the Options button and turning on Share files and folders using SMB. Then in the Finder menu selecting Go…Connect to Server… and entering smb://ComputerName for each. This doesn’t seem to be permanent between reboots however.
Here is a thread discussing this issue.
One suggestion from the thread is to go System Preferences…Network and select Assist Me at the bottom. Trying to setup a connection sets some values (DHCP?) which may fix things.