One simple thing you can try is to reset your custom config.wtf files. Sometimes they change things in major patches that happen to break some custom settings. Try renaming your WoW\WTF folder to something else (e.g. "WTF-20120829"); that way you still have everything and can put it back, but you can rule out anything to do with your personal setting.
If that doesn't help, we need to gather more information about the problem. When this type of error happens, there may be a new entry in the Windows Event Viewer with some technical information that can narrow it down. Type "event viewer" into the search box in Windows (e.g. in the Start menu) to easily find it, and open it up. Then in the left pane expand the "Windows Logs" folder, and select "Application" under it. It might take few seconds at this point, but it should bring up recent "events". You'd be looking for an "Error" level of event, from the Date and Time of that message, from the source "wow".
If you find an entry like this in the list, select it. Then in the bottom half, pick the "Details" tab, and click on "XML view". You can copy and paste me that XML and that might help identify the problem.
Lastly, ISBoxer comes with some diagnostic options under the Help menu. In this case, pick "Diagnostics for game crashes" and follow the instructions. It does say to launch the game WITHOUT IS/ISBoxer, and get to where it would normally break (e.g. at the login screen?). Once it does the "Copy", go to
privatepaste.com and paste it there, click the Save Paste button, then give me either the link to it or the paste ID.