Hi guys!
I can help with this, I ran the Norton Internet Security group until a few months ago.
All web traffic and e-mail traffic goes through SymProxy so it can be filtered. Like Snefen's excerpt says, it can occasionally read as consuming 99% CPU because of badly formed scripts on websites, but it's not actually taking that much CPU time and was fixed right after we saw it.
When you did the uninstall reinstall, you probably did a LiveUpdate at the end and got the newest version of the proxy that fixes that bug.
Sorry for the inconvenience! We really work hard here to prevent problems and deliver quality products, but things like that crop up in the field sometimes so we try and fix that stuff ASAP. Heck, we run the software on our own machines too, so even if we were lazy jerks, we'd still want it fixed right away.

I'm in charge of SQA for Norton SystemWorks this year. It hasn't gotten a lot of love for a while, but we're doing some exciting stuff with it this year that should be pretty neat. I'll tell y'all about it when we ship.
Regards,
Chairboy