That info is appreciated and saves a bunch of Q&A. I am starting to like that method of information request.
I do not use Symantec but looked at their Support section and with the 10.X Corporate Version, there are a lot of options that seem to be enabled by default that are going to slow any system down.
You may not need all the options enabled if you are working at home, or if you feel compelled to use all the protection features, accept that your system performance will be suffer.
I suspect Symantec as a contributor to your issue, but since I do not have it installed, I can't tell you how to turn it off entirely or just parts and pieces. They have a support forum, but I did not see a FAQ about "how do I turn all this stuff off to see if my system performs better".
I think Symantec has some good stuff and they have products for home and corporate environments. Personally, I would never use Symantec unless I was forced to (by Corporate).
If I was sitting in front of your system I would click Start, Programs, find Symantec and look through it to see what is enabled and decide if it is appropriate. Any kind of "real time" scanning, anything with "all files" or a separate Symantec firewall that conflicts with the built in Windows firewall. Two firewalls generally equals performance issues.
If you start messing around in MSCONFIG, you may get their eventually, but I am not a trial and error fan.
Turn some of the "all", "real time", settings in Symantec off, reboot, make sure they are still off and see what you think.
You could read the documentation, but I always save that part for last