How can I remote desktop from Windows XP into Windows Vista?

You need V6 of Remote Desktop

You need to have the current version of the Terminal Services Client (RDP 6.1), which requires XP SP3.

Also try to enable downlevel access from the Vista box:

Start -> Computer -> Properties -> Remote Settings -> "Allow connections from computers running any version of Remote Desktop (less secure)"

You do need either Vista Business, Enterprise, or Ultimate too. Though I think there are ways to enable it on home, but it may void your license...

I know you said you didn't want VNC, but CoPilot does work very well, and is quite agnostic about windows versions.