Computer crashes every time I try to unrar anything of substantial size

You don't saw what the nature of the crash is, but it sounds as though your rar prog. is corrupt. Not enough to prevent it from running at all, but enough to cause a major system malfunction. I don't think that your problem is due to memory shortage, if that was the case, I think that it would just freeze. Try uninstalling and then reinstalling the program.

check any or all of the following: -try re-installing the program -your compressed file might have virus or corrupted -you're running out of memmory -you're running out of disk space -re-install OS -check if there's an error message