I had this exact problem too with my MSI HD2600XT 512MB Silent edition card, but managed to resolve it in PowerDVD 7.3 after installing patch 3104a.1 from Cyberlink's website.
Here's a direct link to the patch:
Updates - CyberLink PowerDVD Ultra - Hi-Def Disc Playback
For more general patches available for PowerDVD, try
Updates - CyberLink PowerDVD Ultra
After installation, CPU usage has dropped from a constant 100%, to ~20-30% whilst playing HD-DVDs. It also seemed to resolve other PowerDVD stability issues on my AMD 4200+ X2 32bit Vista Ultimate setup.
Hope this helps!
Ben