I have been using Norton Antivirus/Internet security for years now (on 3 PCs running XP) and the big turn around for the past few years with 'light' software has been great. Most times, issues have been due to non-removal of other/earlier versions of Norton software.
Installation: The recent improvements in installation are continuing to be great. I was able to upgrade from NIS2010 to 2011 in under 2 minutes. If you still have license period left from your previous installation, choose to upgrade using Norton upgrade option (You'll get pop-up notice or check Support - Check new version/product update). If you upgrade using the disc, it will over-write your existing license period (doesn't make sense, but that is the case here). However, after internet upgrade, you can enter the license code from the disc to extend the license for another year (just before your license period expires). During installation, you may notice wireless connection disconnecting a couple of times, annoying, but doesn't affect anything.
Performance: There have been slight improvement in performance this year. The virus signature and upgrades are downloaded in small size files in the background. So you do not feel it running in the background even in some older computers. I am still using WinXP and therefore cannot comment on Win7 compatibility.
Security: I haven't had attacks that damaged anything, but Norton did report preventing several Trojan-type attacks. If you or people that you share the computer with connect to unsecured wi-fi networks, often visit download sites or click links from email or visit other potentially unsafe websites, you should set the update scheduler to constantly look for signature updates rather than schedule signature updates at computer idle time. This is going to take sometime and perhaps more resources, but definitely enhances security.
As in previous versions, the Firefox and IE companion generates a browser toolbar which prompts sites of potential danger (phasing sites etc.). It also identifies most of the secure websites (amazon, credit card, bank websites, hotmail, gmail) as safe websites.
Norton has the option to use their password manager, but just like last version, the password manager is not so secure. Basically you can store your passwords and access them using a single password for the manager. However, occasionally, you can login to password manager by clicking cancel and not entering the password!!
Norton uses a website-based parental control option now. Despite being called a different name and being able to set profiles for family members, it is not reliable and not well thought-out. It blocks Google claiming that it is a discussion website, but allows yahoo without any problem. Similarly, occasionally you can get in to blocked websites just using the 'back' button in the browser.
Overall, it is still a great piece of software for protection against internet threats, but password management and parental security may not be Norton's forte.