Make sure you have checked the 01 - Pre-requisites checks before System Requirements section before attempting the installation.
In the %REP_HOME%\Rep Attack 11g XE directory, double click on the VirtualBox installer file called VirtualBox-<version>-Win.exe, and follow the instructions. You will need Administrator privilege on your computer to install VirtualBox.
The example shown here is installing VirtualBox 4.3.14 although version 4.3.12 is distributed with Rep Attack and works fine. The installation should only take a few minutes at most.
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