Your WinTV card has two TV tuners and two TV signal inputs, but that does not mean that one input goes only to one tuner. On the back plate the top F-type cable input is for the FM radio tuner. If you plan to use that, the input could be from any FM antenna. IF your Comcast cable service includes FM signals, a 2-way splitter will allow you to connect it to both the FM and TV inputs.
The second one down is TV In. It is where you connect a cable TV input, like from your Comcast box. This source may have two types of TV signals: older analog TV, or digital TV coded in the ClearQAM format.
The third one down is ANT In and it is for your own external off-air antenna if you have one - NOT for a cable TV input. This signal may have two signal types also - older analog TV if there is still any around your area (no longer likely) or digital TV encoded in the ATSC format.
This from the Hauppauge Installation Guide:
Plug your cable TV into TV Connector. Plug your ATSC digital TV antenna into the ATSC Digital TV input.
Note that there are two digital formats here - one called ClearQAM on cable, and another called ATSC from an over-the-air broadcasting station picked up on your own antenna. The come in on different input connectors.
In your card there are two tuners, and each has access to both input connectors and uses the most appropriate one. One tuner is spec'd as a "125-channel cable-ready tuner", which mainly will deal with non-digital channels from the "TV In" cable input. I would suppose, however, that the WinTV application and probably WMC have a way to let it look at the "ANT In" source for non-digital channels, too. The other tuner is specifically for digital TV signals, and it can deal with both the ClearQAM ones on the "TV In" connector from cable and the ATSC ones on the "ANT In" connector from your external antenna. So unless you have your own external antenna to plug into the "ANT In" connector, don't use it. Do NOT connect a second branch of your Comcast cable to this port.
First thing I'd suggest as a test: take the cable from your Comcast "Coax Out" port that is now connected to the back of your computer and connect it directly to the cable input of your TV. Take a look at all the channels that seem poor when run though the computer. Are they still poor direct to TV? If they are, the signal strength on the cable is poor and you may need advice from Comcast. For example, were you using your Comcast service and box this exact same way - direct to the TV input - before you got the computer? Was it bad then? Or was it OK then, and bad now? If it changed, what did you change that might contribute?
If the TV gets a good signal direct but it's poor from the computer, make sure the Comcast cable is connected to the "TV In" connector on the back of the computer (2nd one from top). Within your software, check that it is using that input port when you try to get either analog or digital channels.
Next thing to check is this hint from that Installation Guide's Troubleshooting section:
Problem: Poor Reception or Not detecting all channels:
Poor TV reception is generally the result of an weak TV signal. When using a 'roof-top' antenna, an aerial booster may be required if there is poor TV reception. Also, in some areas using cable TV, a format called Cable HRC is used. If so, try rescanning but use CABLE (HRC) instead of cable in the Broadcast/Cable box.
I don't know if Comcast uses this system, but try it anyway to see if it helps.
Regarding recording shows, there are a few variations. I'm certain there is a way to record any channel when you are not watching something else. But suppose you want to record one channel while watching another. With two tuners I would expect that is OK with a slight restriction, but CHECK YOUR DOCUMENTATION FOR THIS. I expect that you can do this only if one is digital and one is not, since the two tuners each are dedicated to one signal type only. Similarly, you might be able to record two channels at once with this same restriction. Then there's a third possibility - watching some stored program (file) as playback while recording new program(s) from one (or two?) channels. I don't know if the card and computer can do this.
Let us all know what you find out. Many of us are in the same boat and dealing with the same problems.