I agree with a lot of what gerryf says but...also disagree.
Some people think I am a little paranoid because nothing significant has happened to them yet, but a major part of my job is Network Security so it is a risk associated with the job I guess. I SEE what is out there. I hear about information and identity theft on mass scale. I am not going to say most home users are immune or strongly immune to viruses. It is just not true. Most are very susceptible, the reason there are fewer mass outbreaks is that people are slowly getting smarter about computers - not that computers are getting more secure. People now know to keep antivirus protection up to date. IF what jerryf says was true, spyware would not be an issue either – spyware gets to you exactly like viruses do. There are two methods most viruses are spread today – both involve unpatched computers. So number one – patch your computer. The first way is having an unfirewalled connection – dial-up or broadband it doesn’t matter just takes a little more time on dial-up. The second is by opening virus containing e-mails…TURN OFF YOUR PREVIEW PANE in Outlook and Outlook Express. Even if you know enough to not open the e-mail, by just clicking on it you open it if you have the preview pane open. This also attracts a lot of SPAM to you. How? Because if you open a spammer’s e-mail, usually they have some sort of confirmation in the body of the message that you indeed DID open it. So they now know you received it and opened it making you a more popular target. But even when your remove these methods – it doesn’t stop much of what is out there.
Destructive viruses rapidly faded in popularity with virus/malware writers because there is no benefit to the writer - in fact it marks them as wanted individuals. Then came the worm writers, just to spread the infection far and wide as fast as possible - again marked by the law. Now many have instead turned to hacking and a bunch of other types of software we can lump all together as "spyware" for this conversation. Stealthy stuff that makes them money and something that most are not getting in trouble for presently. They can track your activities, collect information about you (name, CC#s, bank information, user names & passwords = identity theft), take control of your PC to use for their own ends, etc. An un-patched, un-firewalled computer on a broad band connection has been compromised on average within 4 minutes, often within 30 seconds. In my experience the ONLY Antivirus package that looks for any spyware is Norton Antivirus. I have been running Computer Associates eTrust and AVG on separate computers for about 2 years, I get tons of trojans. I am one of the safest surfers you will find. I NEVER open suspicious e-mails. If I think there is the slightest chance they are legit and have any importance, I will look at the code and see if I can determine the contents from that.
Any 5-10 pieces of anti-spyware are not enough to totally cover you. Anti-spyware is FAR behind anti-virus software. However, AV software is still a necessity. And the really bad thing is that it is not some one person attacking you - mostly it is compromised systems that are doing the probing for other vulnerable systems.
If you are on broadband:
#1 MUST have a HARDWARE "Firewall" router. Stops 95% of the easiest way for hackers to compromise you and stops network virus spreading cold. I would add a software firewall as well. MS's SP2 firewall is still a joke. Try one of the free ones out there - anything is better then nothing.
#2 A GOOD AV solution. Personally, I am not worried about viruses. BUT I have 15 years in IT knowledge and good common sense. I have had viruses get by virtually every piece of AV software I have seen in the last 2 years (eTrust, McAfee, Norton, Trend, Panda, etc – I see them all). I only recommend Norton – fewest failures and best overall protection. Someone mentioned "Symantec Antivirus" and that it is different from Norton - I think they found something good to smoke. The only Symantec AV that is not named Norton is for Symantec AV for PDAs - NOT for PCs. That would explain why they had nothing running…
#3 Anti-spyware. This is the REAL threat these days. You can take ANY 5 pieces of Anti-spyware (AS) products and they will NOT remove 100% of the 135 most common pieces of spyware. My personal arsenal at the moment is: Adware SE (Free), Spybot Search and Destroy (Free), Spyware Blaster (Free), Microsoft AS Beta (Free), Webroot's SpySweeper (Free 30-day Trial), Computer Associates PestPatrol Enterprise (Free Trial). Hijack this is also in my list of tools - for the items not caught by the above. It helps me identify and manually remove spyware. I just added the MS AS Beta to my list this week. It looks to be a good product and Giant was actually the most effective product prior to thier aquisition by MS a couple weeks ago.
Adware SE (Free), Spybot Search and Destroy (Free), Spyware Blaster (Free) - These programs miss most Trojans and Trojan downloaders. Yes that is right, there are Trojans out there that's sole purpose is to phone "home" and download the latest spyware that "home" wants on your computer. I don't know how the MS beta performs in this area yet, but my best solution for this is PestPatrol.
If the anti-spyware program is not in my list, then look into it thoroughly before installing it on your system. Far too MANY are actually spyware themselves. They even advertise in banner ads – but when you look into them you find out the truth often.
If you are worried about the processes from Norton, how about the trojans and spyware you don't see in your process list and all the network traffic & CPU cycles they are eating up? I would not be worried about the slight slowdown you might possibly get from the product. If you want an ultimately tuned gaming machine - then it has to be ONLY a gaming machine. (no e-mail, no surfing, no nothing) and you can protect it pretty well with external firewalls and being fully patched.