virtualbox.org

It's impossible to uninstall Virtual Box 3.0 when using a Vista's Standard User (not admin):
UAC asks me for the admin password, I type the password, the uninstallation process starts but, when it reaches the end, the uninstallation process is aborted (the msi's progress bar is emptied).
3.x version is affected, old versions 2.x were not affected

If you know an administrator's password, then there's nothing stopping you from logging into an admin's account and uninstalling it from there.

a well designed software should allow to be installed and uninstalled with a Standard User via UAC elevation, without needing to be logged in the system with the administrator account.
Please follow the "Developer Best Practices and Guidelines for Applications in a Least Privileged Environment": http://msdn.microsoft.com/en-us/library/aa480150.aspx

That's a nice page you linked to, but you forget a thing. UAC is not perfect. There are still quite a lot of bugs in it and I consider your situation one of them. Did you test UAC privilege elevation with a 2.x.y release recently, or did your system change in the mean time, like Windows Updates etc.?