Here's the install guide for the MPV decoder:
I think I used this decoder under XP and it worked pretty well.
I did some channel change testing on my test box and there is one channel from my cableco that tends to have a "messy" stream that has a slow channel change (but also does not play smoothly). My TVs have no problem with this, but NPVR does.
HTPC: E7500, Nvidia 9500GT, ATI Theater 650Pro PCI/Firewire Network Recorder/Avermedia Duet A188, NPVR 2.6.2, Win7 Pro
Test: Asrock 890GM Pro 3, Athlon X2 255 , internal HD4290, ATI Theater 650Pro PCIe, NPVR 2.6.2, Win7 Home Premium
Lounge: Athlon 3200, Nvidia 7600GT, Network Recorder, NPVR 2.5.5, Win7 Home Premium