I'm a new user of Fedora 19, which I'm very satisfied of (despite of what will follow). However, after following some advice on installing programs, changing themes and putting shortcuts on the desktop (still can't figure out how), like gnome-tweak-tool and alacarte, I started to have problems when I wanted to restart the gnome shell (alt+F2 - r + return): on the application menu when I go to "Utilities" or "Others", the graphics become messed up (fragments of white/yellow pixels, messed up fonts, messed up pictures... pixel chaos). There are still some graphical glitches left in the taskbar after this. But most of it, Firefox and some other applications don't work anymore.
I really don't know what I've done, but I can't use the system normally anymore. And I don't know if I can reinstall it.