I don't do Google music either. It worked ok when I had it (nothing great or negative). I prefer using the Foobar to stream to my AVR. It used less resources and works every bit as good.
I joined AVS because you seem to have the same problem I do. Chromecast is working mostly well for me, but I still have a problem. I can cast from my Android device, but I'm denied from my Win7 laptop.
I'm not sure Symantec is the problem, but I can't figure out anything else. To troubleshoot, I've added a half-dozen exceptions to my Symantec Endpoint Protection client. None of them help. I'm administrator of my laptop, but it's locked down by the group policy. I cannot kill the SEP service or client, even from Safe Mode...
If I use my Android to get my TV 'Ready to cast,' my laptop still cannot see the device. I've called Google twice (what a disappointment that has been,) and now am waiting for the second tier to get back to me. I've tried this with two Chromecasts, and two TVs, but I don't have another wireless PC to try.
Since someone else posted the same problem you and I have, but no solution, and Google's search results for Chromecast and Symantec are limited, I'll post a reply here in case Google are able to help me finally get connected.