Code Completion does not do a thing on Vista. I set the preferences for PyDev, but when I try to use the feature, it simply doesn't display anything. There isn't a python.exe process open, as I suspect there should be. I tried running eclipse as Administrator and in windows XP compatibility mode, to no avail. The only good part is that it doesn't hang either, as some others' installs seem to do.
Do you have something in your error log?
Does it work in some other editor (e.g.: java)?
Nothing is logged. If I do it with Java, the window does come up when I hit ctrl-Enter, but not when I hit '.', and it never has any suggestions. I think I'll just move to the Visual Studio "Orcas" which I finally finished downloading today. I'm sure that was tested on Vista :)