I'm a Sys Admin running Windows 2000 + the Norton's Corporate AntiVirus, AntiSpyware, AntiVirus for Email, Web Proxy, etc -basically everything in the suite and I've never had a problem with it. This isn't the home version you'd be running, but I've never had a problem with their home software either (on my Mom's machine). The only bad thing I can say about Norton is that their Anti-Spyware protection is not so good. But, there's free software out there for that anyway...
All antivirus software will consume system resources.
For anti-spyware: Ad-Aware and Spybot are both very good and free for home users (actually Spybot is free for businesses too, we use it in addition to Norton).
Ad-Aware: http://www.lavasoftusa.com/software/adaware/
Spybot: http://www.safer-networking.org/en/index.html
Just remember to update this software and run scans regularly. Spybot has an option called "Tea Timer" that will alert you whenever a system change is made. This is very useful if you don't make many changes on your system. If, while surfing, you see a window pop-up that says the computer is attempting to change your homepage (or whatever) and you didn't do it, you can "deny" the change and save yourself from having to clean up the changes later. The downside to running "Tea Timer" is that it' pops up a lot of alerts about changes when you install software or make browser changes, etc, so it can get annoying if you make a lot of changes to your computer often. If you don't want it, though, just don't turn it on. -It also has an "Immunizer" that recognizes the most common spyware and just silently blocks it all.
Anyway, if you go with either of these and you have any questions about the download, installation, or configuration, scan results, etc. PM me and I'll help you out.
Like others have said, make sure you run Windows updates as well. This will help more than anything else, especially if you have a high-speed connection. The easiest thing to do is just turn on Auto-Updates. Usually found in Start > (Settings) > Control Panel > Automatic Updates -then select "Automatic" and pick a time that you want it to download and install them. 3am is the default.