Sometimes it happens that a Wine application crashes (slowing down the system and making it almost unusable).
In most cases I'm able to kill the program with xkill, but sometime I've to restart as Ubuntu seems not to respond very well (the only thing that works is ALT+F2, the launcher; xkill doesn't).
I've tried to use wineboot -r or -f but they don't seem to work very well..
If something is unclear, let me know I'll try to explain better :)