Some of the TV channels on my spectrum.net are pixelated only on my desktop computer.

Yeah, most video streaming uses an "Adaptive Bitrate" encoding scheme.  The server and client measure the quality of the connection between the machine and the video server, and they adjust the quality of the video down to something that will play under the available network conditions.  The more bandwidth is available, the better the picture, and vice versa. 

This one is a little weird, so I'd start with "When in doubt, reboot".   If the issue persists, then I'd look at why the network connection to the desktop is problematic.

It's interesting that only the wired machine is problematic (I'm usually telling people to plug into ethernet to troubleshoot spotty wireless connections), but you can do the same experiment.  If the machine doesn't have WiFi available, you might try a different ethernet cable, and/or plug directly into the modem to eliminate anything between the machine and the connection to the outside world as a possible cause.

There's a lot of stuff between you and the server that the video is delivered from, but much of that exists outside your sphere of visibility or control.  That other devices are working fine on the same connection makes me think that you probably have decent connectivity (although it might be that other devices on your network are hogging the available bandwidth, so you're left with a sub-par stream on the desktop because there's overall network contention).  Hooking directly into the modem means that you're not sharing the connection with anyone else.  If things still look bad, and you're doing an apples to apples comparison with another device on the network, then you have an interesting puzzle.

i know if the Internet is slow you get buffering and low quality pics.the pixeling did you say that none of the other online streaming devices pixel ? i would try to update you flash and if just that computer still does it spectrum is on facebook. Find there page and you can actually message them live and they can go through all the trouble shooting for you .  I was a Timewarner technician for 7 years . Hope you figure it out cause usually pixelation is cause by data being messed up from some sort of interference but that only happens on the cable boxes now or hd antenna's . Good luck