PLAXIS 2012 versions and earlier of PLAXIS 2D and PLAXIS 3D have not been designed (and 100% tested) for Windows 8. At this stage, we can not guarantee compatibility, correctness and accuracy of the mentioned PLAXIS versions under Windows 8.
Starting PLAXIS 2D AE, PLAXIS 2D can be used on a Windows 8 machine. See the product specifications page for more details: System requirements
PLAXIS 3D 2013 can be used on a Windows 8 machine. See the product specifications page for more details: System requirements