I was testing Cardspace which comes with IE7, .Net Framework 3 or Windows Vista and while selecting a card the "Windows Cardspace" application hung. There is no way to kill it using Task Manager because your desktop and all other processes are inaccessible when using this tool.
If your computer is on a network, you can kill it remotely:

Open a command line and type
psexec \\computername -u username - p password cmd.exe

Provide a username and password for an admin user on the remote machine.

You can download psexec for free at www.sysinternals.com

You now have a command line running on the remote machine, type:

taskkill /F /IM icardagt.exe

This should kill it, you can then type exit to close your remote session.