That should be the correct format then. Have you considered the firewall on the PC? Try turning it off and see how you get on.
There's also a firewall on the Mac too.
But as Mike said.... only
temporarily turn it off !
It might be that you need some third party solution on the Mac such as Thursby DAVE to make the leap from Mac to Windows, because I'm not sure how many of the Microsoft authentication algorithms were built-in to OS X 10.3.... but keep on trying with the built-in solution first.
The other option is to install FTP server software (freely available) on the Windows machine and access that from the Mac.