Wow, you all ready expect windows vista to work perfectly right out of the gate? After doing some reading up I think the best bet is just to rent one from the cable company.
For a pc being used as a pvr sata drives, windows vista, and amd 64 bit cpus are either overkill or not what's needed. Windows media edition, a 2.8 p4, a gb of ram, 200gb ide hdd, dvd r/rw driv, a nice capture card, and vid card are all that is needed.
Don't get me started on the drm of vista, personally if I was building/buying a pc as a pvr I would stay away from vista.