From time to time, while browsing your XBMC library of files you may get the prompt, “Need to unpack a big file, continue?” This means XBMC has a large rar file it needs to open, so large that it may take a while/affect your system’s performance. Typically you can just answer yes and not have to worry about it again.
In my experience though you get this error when you have a corrupt or password protected rar. Answering yes gets you an endless loop of prompts, answering no will result in the prompt going away only to come back the next time you open your library.
What to do?
Via what ever method you have, go to your <xbmc_dir>/temp directory. In there will be the offending rar file, possibly in a directory called rartemp. Delete any rar files and any directory with rar in the name. Restart XBMC.
The next time you open your library you should be all good.
I use an SQL server for my library, sometimes I have to go an extra step and delete the file’s reference in my xbmc data. Look in myvideos->files and find the recent entry with rar:// in the strFilename column.