My setup: Windows 7 build 7100, Hauppauge 2250 dual tuner with the cable plugged in directly from the wall.
My problem is that my WMC does not detect/display all the channels that my set top box displays. WMC picks up the channels up to about #98, including several QAM channels - that part works great. But when using my cable box I get channels from 100-200 and WMC doesn't seem to be able to detect them. For example, my set top box display History-International on channel 116, Fox Reality Channel on 118, DIY network on 121, but I can't see these using WMC.
Cable companies encrypt a significant number of channels they offer. The cable box acts as both a tuner and decrypter, so whatever tier package you subscribe to, above basic cable, requires the set top box. Since your TV Tuner card does not have the decryption capabilities of your set top box, you're only going to see those channels set unencrypted.
If you check out your cable companies channel listing, you'll probably see that the channels you see with your TV Tuner are the same channels included in the cable companies basic cable package.