I'm sure I'm too late on this thread to be useful. If I'm not, then I'd like to second what SeaRide says. Basically, to get the best signal to the modem, you put a splitter on the cable where it comes into the house (or even before the cable enters the house). Run one of the two sides to your existing TV network, and the other straight to your cable modem.
I got a cable modem several month ago, from Comcast. Essentially I did all the work myself, running a new length of double-shielded cable into my computer room, fishing the wire down the wall, etc. I did this because I heard that the installer wouldn't "fish walls". When he arrived, he said he woulda done it himself no problem. Oh well.
And honestly, your cable modem doesn't have to be right next to your PC, as long as the output from the cable modem is an ethernet wire. It simply needs to plug into the cable and a power outlet. The ethernet wire then runs to your PC. If you use USB, I think you have to have the cable modem within 15 feet (the max length for USB cables).