ctrl+shift+z can fail to function for other programs after uninstalling an old version of mudlet.
If restarting your computer isn't a viable option or fails to fix it, then this should fix it:
1) Download an old version of mudlet e.g. mudlet-1.1.0.exe (http://sourceforge.net/projects/mudlet/files/)
2) Install it. Leave the defaults be. That's probably how and where you installed it the first time.
3) Find the mudlet shortcut on the desktop. Right-click it.
4) Choose properties.
5) Clear the shortcut box (it will have CTRL+SHIFT+Z in it) by clicking in it and then pressing backspace.
6) Click Apply/OK.
7) (optional) Test to make sure ctrl+shift+z doesn't bring up mudlet.
8) (optional) uninstall the old mudlet version.