I'm on Jaguar, so with that in mind...
To see my Windows boxes I just hit Cmd-K in the Finder (or 'connect to server' in the Go menu), then twiddle my thumbs for a while (minute or more) until it finds the workgroups on my network. They're (eventually) all listed in the Connect to Server dialogue.
To put your mac into a different workgroup, go to your Applications, then Utilites, then open the Directory Access app. Unlock the settings if you need to, then go to the Services tab, click SMB, then Configure. It'll then bring up a sheet where you and change the workgroup you're in (and lets you specify a WINS server)
There, easy! (Yes, I know ... NOT!) I had to look up this 'change workgroup' thing in The Missing Manual.