Microsoft says they do not have enough information from WRI to ensure that Mathematica V.8.01 is fully compatible with Windows 8.1. They recommend asking the software provider before upgrading. Mathematica 9 does not appear in the list of full compatible software for W8.1 either. Only Mathematica 9 add-on extras appears in the Microsoft W8.1 compatibility list.
Might I ask whether anyone Is running Mathematica V8 and/or V9 on Windows 8.1?