The Panel is either lxpanel or tint2 depending on which one your using. If it's Openbox that's crashing, you won't have Window decorations anymore nor be able to access the right click menu (if not overridden by pcmanfm). Anyway, this narrows dow the potential culprits.
This means you'll have to open a terminal window (sakura or xterm) and start pidgin (use pidgin &) in there, then just do what crashes pidgin. It might throw an error as to what is causing the crash or might not. If it does, just copy down the error and paste it here. If it doesn't give an error, then it's the panel's fault. You'll have to type lxpanel & in the terminal window and then start pidgin again (with pidgin & like before). Now crash it again. That will most likely give you an error. So copy it down and paste it here. Once we know what's causing either of the two to crash, we can start tracking down how to fix it.
My best guess? It's probably a missing dependency. So check that libnotify, pidgin-libnotify and libsexy is installed. If not, install them. That's the only thing that occurs to me that can kill both a panel and pidgin like that.