Nor-r-r-ton!
The Norton Consumer Suite of antivirus/antispyware/firewall/etc. eats up LOTS of memory and CPU cycles. It will slow your system down 10-15%. When you run an application that wants lots of resources, it will slow to a crawl.
Another performance drag is Microsoft's Defender anti-spyware. It's not nearly as bad as Norton until it starts running a automatic scan, which is scheduled automatically during installation. If the computer isn't on at the scheduled scan time (2AM?), a scan will run whenever it feels like it, and your system will come to a screeching halt.
Norton corporate isn't nearly as bad as the consumer product. For my money, I'd lose Norton and go with Computer Associates (my-etrust.com). Much better product, more frequent updates, and doesn't degrade system performance nearly as much. If you're behind a router with a built-in firewall, I'd skip the firewall and live with the router/Windows firewall combination.