The on-line product features & system req's pages state that Windows 7 is supported, but no mention is made of Windows 8.
See:
http://www.piriform.com/recuva/features/full-windows-os-support-and-many-languages
http://www.piriform.com/docs/recuva/introducing-recuva/system-requirements
However, other users do mention installing it on their systems while posting to this board for help.