From the Control Panel start the System applet and go to the Advanced tab:
Click on the Environment Variables button to open a dialog for editing variables:
Environment variables can be added for all users or only for the current user. For OWLNext root folder it is better to set it for all users.
Click on the New button in the System variables group:
Enter the name and the value of the variable and click OK. Depending on the application which will use this new variable, you may have to restart the application or the operation system.
Wiki: Setting_up_C++Builder_XE2
Wiki: Setting_up_Visual_C++_2005-2008
Wiki: Setting_up_the_Borland_C++_IDE