Hello Didier,
I'd suggest to use PowerBuilder 12.6 as it is the first release which supports Windows 8:
System Requirements - Installation Guide - SAP Library
For your information, PB 12.5.2 has not been tested against Windows 8:
It would be very difficult to help you on your application failure if you did not detail the specifics of this failure...
Anyway, for migration considerations, please take a look at this wiki page:
Migrating PowerBuilder Applications - SAP PowerBuilder - SCN Wiki
I'm sorry but don't know what the price is for a standalone PB 12.5.2 or PB 12.6 release. You may involve sales rep (SAP Support & Maintenance | SAP, then contact us)
Yo can go to this url to get a trial of PB 12.6:
http://scn.sap.com/community/developer-center/powerbuilder
P.S.: by the way, it is better to use this forum to post your technical questions as there is a wider audience
HTH
Jacob