ET3D: It all depends on what "works for you" means. If buying something new upgrades your experience and you think that's worth the price, then by all means junk the old one and move on. It's a good thing too because that's what makes technology advance. If nobody wanted to buy better tech we'd be in a rut.
I know that for many people that "ancient piece of junk" as darthspudius called it may be all they feel they need, because they don't know better, but put them on a new system with an SSD and a decent amount of RAM and they'll realise that there's really no need to wait for the PC all the time.
Firebrand9: I keep the resident scanner off and just scan anything I need to
as I need to. This allows my PC to run
MUCH faster.
ET3D: How much faster?
For the first point, I mean this in the context of keeping with what works and not just blindly "chasing the rainbow" when you have something that already meets your needs. XP can play 99% of the games out there and would have full capability far into the future beyond what the naysayers and fearmongerers would have you believe.
I don't disagree with the point on technology advances, but most advancements I've seen over the past 10 or so years have been reinvention (EG - Facebook is really just public email. Twitter is just really stripped down concise-by-requirement public email. Touchscreens are just a cludgy mouse. The list goes on ad infinitum) rather than true innovation. I take umbrage with the trend-seeking that occurs as a result of some old thing being repurposed and people acting as if it's the greatest thing ever, because it's, you know, "new and shiny". In other words, the new technology must add objective value. This describes my sentiments well :
http://www.youtube.com/watch?v=xSSDeesUUsU For the last point I haven't measured it to be able to give you some sort of percentage difference. But, consider the OS is not taking a constant file-reading/writing hit on due to the OS detecting there's been an alteration, updates don't occur at a time other than my choosing resulting in a bandwidth and the aforementioned read/write hit, and less ram and CPU are used on the PC overall. There's no free lunch and AV software are notorious resource hogs. So, if you're asking the PC to do something and there becomes a need for those resources to be used elsewhere (read as : on my demand), you won't be getting the full capabilities of your system. It's as simple as that.
But to attempt to answer your question, the effect is noticeable in a significant lack of lag in anything you do on the PC. A good car analogy is akin to turning on the A/C which forces the car to run an extra belt to the compressor which essentially robs horsepower. Removing unnecessary bs is part of the hotrodding process on a PC.