If I switch from DirecTV to cable, which I'm considering, I'll probably be picking up a PVR-500 or two to use in my Linux Media Center (LMCE). It uses MythTV for the PVR type stuff, but I'm not sure if there is a windows port of the back end server stuff. I know you can use a windows box as a MythTV front end.
However, there IS a way to do this. If you use VMware or some other virtual computer you can also pick up something called HDhomerun, a networked digital dual-tuner that supports HD. In Linux/MythTV it "just works" out of the box. As it's networked, it's outside the VM so the linux VM can access it just fine. With that backend running in a VM you can run the MythTV front end on a windoze box and have full control of a dual tuner PVR. Personally, I think the better option is to run a dedicated linux/MythTV server somewhere on your network then just use the windoze/MythTV front end where you want to watch.