I have a similar setup to yours. And a year ago I was also disappointed in the PQ first time I fired up the Inf4 tuner. Be sure to go into the WMC settings and do a proper picture calibration. Also tune in a TV program and right click then go to "zoom" and be sure you are set to Setting 1 (not 2,3 or 4). Finally, right click on your PC's screen and go into the Intel graphics properties and be sure the setting are all ok there. It took me a bit, but I would say the PQ is now comporable to my cable box, perhaps a tad better. Finally look at the TV's settings for the input your PC is using. There may be some that need to be changed such a noise reduction etc.
Assassin HTPC + Echo, Sony 49XBR850, McIntosh MX151, MC202, MC7104, MS300, B+W 803D2, CWM's, Janus 1W Sub