With that setup, you should easily be able to run Aces High at your monitors native resolution with 1024 Textures (with Hi-res download), all settings MAXED and self shadows at 1024 or less. Make sure 'other planes shadows' is turned off. If you're still having performance issues at that point, try turning shadows off completely and see what happens.
Your computer is pretty much 'overkill' for AH, so your bottleneck is going to be either an extremely slow hard drive (doubtful), or a video card that is lacking in memory. The 260 is better than most people in AH play with, and should hold up on its own just fine, but it may not be able to smoothly support shadows.
If you are running a software firewall, I'd be tempted to just disable it altogether, as the router acts as a hardware firewall and will catch most of the bad stuff that is going to come through. As far as Norton goes, you can try uninstalling it and see if it affects performance, but when installed, Norton actually modifies some windows files so the only way to get rid of it completely is to format and do a fresh install of windows.
I still think people tend to overact when it comes to Anti-virus/Firewall protection. As long as you secure your browser (While IE is getting better, I personally still don't trust it. You CAN uninstall IE from Win7. I've done that since day 1 and run Firefox), follow safe internet practices (be careful what you download... don't go to questionable websites, especially ADULT or WAREZ websites, avoid P2P), and don't leave your computer on 24/7 that reduces your chances of infection by probably more than 99.9%.
I personally think malware/spyware/adware is a much bigger threat than trojans/viruses these days and this is software that comes 'packaged' with other, sometimes mainstream, software packages. For example, one of the more common 'free' downloads (I think it's Sun Java but not 100% sure) has a page that shows an End User License Agreement, and underneath it is a checkbox that says "Accept the EULA and install ASK.com Toolbar" that is checked by default. If you don't take 2 seconds to look at it (most people don't) you'll just hit next and not even notice. If you DO take the time to look, it makes it look like there is no way to install this without the toolbar, however you CAN uncheck this box and still proceed.
Paying attention to little things like that during software installs will eliminate 90% of these baddies from getting installed. And before you say anything, I FIRMLY BELIEVE that 90+% of the 'toolbars' out there are Malware in nature. I have yet to find out that actually does anything beneficial or enables features that you can't get by using one or two clicks elsewhere. Luckily, Spy/Mal/Adware is usually relatively easy to remove.
Another good practice is to review your list of installed software (add/remove programs) and uninstall anything you don't use, looks suspicious, or that you don't remember installing.
Personally I'll do manual scans with Malwarebytes or Glary Utilities from time to time, and I have Avast (free standard antivirus) running in the background, which I disable prior to flying AcesHigh or MSFlight Simulator and my Win7 installation seems to be running lean and clean.