If you install Norton System Works it will affect your systems performance but thus far just running the anti-virus I've had zero trouble.
To give you an Idea of what I currently and more often than not always have running:
Windows 2000 Pro
AMD 800 MHz
700 + MB SDRAM
70GB Hard Drive
Currently open and running:
Outlook Express, Two SSH Sessions, Two Shoutcast Radio Broadcasts, 2 IE Sessions, QB, PSP 7, StarOffice which is what I'm posting now with, Seti@home, ECCP, Yahoo Messenger, WebTrends and I think that's about it. OH and of course Norton 2003
Isn't life grand?