See if this helps and note in particular the need to precede the Mac's Username with its current IP address:
Network Access from Vista to Mac OS X
When trying to access Mac OS X (10.4.9) from Windows Vista Ultimate, you may be prompted for a Username & Password, which immediately fails with the prompt reappearing. This solution to the problem which may help.
Go to "Control Panel > Administrative Tools > Local Security Policy" then select "Local Policies > Security Options".
In the right pane, scroll down to "Network Security: LAN Manager authentication level properties" and change the setting...
From - Send NTLMv2 response only
To - Send LM & NTLM responses
(MS article re LM levels - click here)
For Vista Home
Go to Services ("Start > Run", type services.msc and click OK) and scroll down to the "Peer Networking Identity Manager", double click it to open its Properties sheet, click the button to Start the service and set its Startup Type to "Automatic".
When prompted with a box for a Username & Password, enter the Username in the format "IP address\username" and your Mac login password. For example:
Username - 192.168.3.22\mike