Accessing shared files on PC /Leopard

OK, first I'm going to just give you a quick suggestion as it may help you. You might find it beneficial to view the Leopard Guided Tour and the corresponding documents on Apple's site. Also, Macrumors has a healthy variety of guides that you may find useful. Just as an aside FYI.

As for your issue, here's what you'll want to do. By default it's going to try to the Windows shares as Guest or the user you're logged into your Macintosh as. So click your PC in the Shared section of the Finder sidebar, and you'll see a "Connect As..." button. If it says "Disconnect" click that, and then click "Connect As..."

You'll be prompted with a window that says "Guest" or "Registered User" -- Click the "Registered User" button. In the Name field, type in the username that resides on the Windows machine you're connecting to. Ditto for the password box. If you want to remember this login, click "Remember this password in my keychain" and click "Connect." This should work with blank passwords, but you shouldn't be using non-passworded accounts anyway.

Assuming you typed in your Windows username and password correctly, you should see shares that the user on the remote machine has access to read. If you don't, you'll want to confirm the user you're connecting as has rights to read the shares on the Windows machine.