I have Vista 64 and it worked for me: I added the files in the navfilter folder (this folder contains four files: clhdspdecoder.dll, hddvdadvnav.dll, hddvdnav.ax, and hddvdnavi.dll) to c:\program files (x86)\cyberlink\powerdvd8\navfilter. The other files (deskband32.dll, hddvd_x.imp, and js32.dll) were placed in c:\program files (x86)\cyberlink\powerdvd8.
Then I went to all programs\accessories, right clicked command prompt, run as administrator. At the command prompt I typed cd.. then hit enter key, then cd.., hit enter key again, then I typed cd program files (x86), hit enter, cd cyberlink, hit enter, cd powerdvd8, hit enter again ; each time showing me the folder I am in. I then typed regsvr32 hddnav.ax, hit enter, confirmation came up and voila, done.
You should now be able to play hddvds, the funny thing is (and this is one of the reasons why I have yet to buy a blue ray disc) that Ultimate Avengers blue ray does not play with powerdvd8, but it does with powerdvd7.
Hope this helps.
ps. I got mine from alt.binaries.hdtv (I have a subscription to easynews, can't stand the p2p crap), searched powerdvd, and found it, it's 5.11 megs.