Does your Windows networking environment employ domain authentication, or workgroup authentication?

If the former, and both your win32 PC and Linux Samba box are domain members, then I suspect that your concerns may be unfounded.   In the NT domain scheme, hosts are authenticated as well as users, so the potential bad person would have to spoof the IP address *and* have hacked the automatic-regularly-changed machine account password.   So you would be reasonably secure in simply doing an SMB/CIFS copy of your already-encrypted file(s) to the Linux box.

But if you're in a Microsoft workgroup environment, then I'd probably concur with fellow monks suggesting SSH as likely being safer than SMB/CIFS.

Fwiw, MAC spoofing is called LAA - "locally administered address".