You can fake the installation of IE6 by applying the attached .reg file. This is achieved by running regedit (you'd run command 'wine regedit' for wine, the FOSS version of CrossOver. I'm not sure how to do it for CrossOver) and then importing the file ("File\Import..." menu item). All the reg file does is set a registry entry that specifies the version of IE that is installed.
CrossOver provides it's own implementation of IE which is based on Mozilla rendering which should be able to everything Xtreme needs of it. The reason that the check is present in the first place is that it makes far more sense for Xtreme to make use of the existing IE web browsing functionality than ship it's own reimplementation and to do that it needs to make sure it's present.
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