Defaults are insidious things. Windows 7 has a file sharing protocol default that isn't set out-of-the-box. It meant that my XP machine could connect to one another, but not my new Win7 box. It's a simple fix, then again aren't they always:

Browse through to "Local Policies" then "Security Options", then "Network Security: LAN Manager authentication level"
Select "Send LM & NTLM - use NTLMv2 session security if negotiated" from the dropdown.

Source: MS Technet forum

If you're connected to a host with one user name, but you'd like to force it to use another:

net use \\computer-name /USER:user-name

Source: ss64