Microsoft has released a security update for Windows Vista Service Pack 1 RC0

Security Update for Windows Vista Service Pack 1 RC0 (KB941644). A security issue has been identified in TCPIP that could allow an attacker to compromise your Windows-based system and gain control over it.

Security Update for Windows Vista Service Pack 1 RC0 (KB941644)
Security Update for Windows Vista Service Pack 1 RC0 for x64-based Systems (KB941644)