I use both for scanning purposes only. No malware scanner has a 100 percent detection rate that is why it’s good to have a second opinion. Just in case one misses something perhaps the other will detect it. That’s the theory. As you can see from my signature i use the free versions of both programs which means they’re both non-resident, for a resident anti spyware app i use Windows Defender(and only this one). David already mentioned that two resident scanners running at the same time can create problems and he is totally correct there( as usual ;D ).
I have UAC turned on simply because it increases my security and believe me it does. But because it’s damn annoying at times i also use the Norton UAC tool which decreases the amount of pop ups from UAC since you can make it remember what you allowed/blocked. Very cool tool and i always thought MS should have included this functionality in the first place. Now read bellow to get some more detailed info about UAC and Norton UAC Tool.
http://en.wikipedia.org/wiki/User_Account_Control
User Account Control (UAC) is a technology and security infrastructure introduced with Microsoft's Windows Vista operating system. It aims to improve the security of Microsoft Windows by limiting application software to standard user privileges until an administrator authorizes an increase in privilege level. In this way, only applications that the user trusts receive higher privileges, and malware should be kept from receiving the privileges necessary to compromise the operating system. In other words, a user account may have administrator privileges assigned to it, but applications that the user runs do not also have those privileges unless they are approved beforehand or the user explicitly authorizes it to have higher privileges.
To reduce the possibility of lower-privilege applications communicating with higher-privilege ones, another new technology, User Interface Privilege Isolation is used in conjunction with User Account Control to isolate these processes from each other.[1] One prominent use of this is Internet Explorer 7’s “Protected Mode”.[2]
Here is what a developer of the Norton UAC tool has to say about it, he explains how it works. Quoted from BetaNews page.
[disclosure: I am a developer at Symantec and was involved in the Norton UAC Tool. I am posting here because I wanted to clarify a few things regarding the Norton UAC Tool and respond to a few of the concerns posted in comments.]
The Norton UAC tool allows an application to run with silently-elevated privileges only in a specific context that was previously approved by the user with the “don’t ask again” check box selected.
This means that there is a difference between regedit.exe launched from the start->run box, regedit.exe originating from a shortcut double click, and regedit.exe launched from a double click on a .reg file (and the context actually changes with each .reg file), and regedit.exe being launched by an application (malicious or not).
Given the contextual awareness of Norton UAC tool’s automatic answering, the Norton UAC tool provides a usability improvement over Vista’s default UAC prompts, while maintaining obvious security improvements in the Vista kernel (such as isolation, file/registry virtualization, and user interface privilege isolation) that are all disabled when UAC is disabled.
We decided to write this tool after we noticed two alarming trends with UAC. The first is that users fully disable UAC - which is a horrible workaround to a minor usability issue (since it disables isolation and virtualization - which in turn removes IE’s protected mode). The second is that users get so used to responding to UAC prompts with “allow” that the prompts are often not even read by the user (Chicken Little "the sky is falling syndrome).
As a result, we are collecting information on the subject matter of prompts in addition to the response times to determine if reducing the overall number of prompts (by allowing users to remember their answers) causes users to spend more time reading the prompts… Microsoft records very similar timing and response information for all of Vista and Office when you agree to take part in the Customer Experience Improvement Program.
As for the impact to your system, the Norton UAC tool produces no running processes and is only active during a UAC prompt. We worked very hard to ensure the Norton UAC tool as as fast or faster than the built in Vista UAC prompts
EDIT: Link for Norton UAC Tool in case you decide to use it.
http://www.nortonlabs.com/inthelab/uac.php