Now it’s known to affect more than just XP. “Windows 2000, Windows XP and Windows Server 2003 are vulnerable to a denial-of-service attack…”
If you have RDP enabled, you are at risk. This includes anyone who uses Remote Desktop Sharing, TS or Remote Asistance. Remember that it’s enabled by default in XP Media Center.
“Until a patch is available, Microsoft suggests users block TCP port 3389 (the port used by RDP) on their firewall, disable Terminal Services or Remote Desktop if not required, or secure remote desktop connections using either Internet Protocol Security or a virtual private network connection.”