One thing I've learned over the years, this subject is very diverse. Where one AV product works well for one user, it doesn't work as well for another.
I'm running Norton 360 on 2 Windows 7 x64 PC's with no issues encountered as yet. I'm not seeing much of an impact with system resources usage from Norton. That said, I'm not a PC gamer, just a basic user, 'net, etc, and I have what's probably considered an average system with an Intel i5-650 CPU (dual core) and 16 Gb Corsair RAM.
Norton is my 3rd AV product installed since I began home 'net PC use in 2004. I've used "Trend Micro" and "ESET" before Norton, with ESET being the previous AV product used.
My experience with ESET wasn't satisfactory but that in no way should be considered a bad review of the product since this topic has a very wide range of recommendations and AV performance feedback results, and from what I've read during the past several years appears to be considerably individualized depending on one's particular PC setup and user activities.
I usually allow 2+ years to evaluate an AV's performance and it's always, for me, going to be graded on how many malicious intrusions breached the AV's defense mechanisms.
For example,. when I was using ESET, I averaged about 1-2 intrusions in the 2.5 years I had it installed but I have no idea as to the details of why the malicious intrusions breached ESET's defenses.
What I do know is that in all cases, no e-mail attachments or questionable surfing practices were involved. I was visiting one of my daily reputable 'net sites and got hit with one of those "FBI" malicious variants, complete with the intimidating-sounding FBI voice over the speakers . Recovery was easy (cloned HDD install) but that had been the 3rd time that something had gotten past ESET's defenses. So I decided to try Norton AV for a couple of years.
Regarding the XP question, I have to make that same decision with my Mom's HP Pavillion Slimline PC. I installed Norton on her PC over a year ago and all's ok but I'm not sure what to do when April gets here when MS drops security patch support for XP.
Mom's also running Office 2003 as am I so that's another issue that will come up in April since MS is also dropping support for it at the same time as XP.