Through many years of building PCs, I've found - for Windows Operating Systems - the following setup works especially well:
Running Resident These run all the time your machine is on, giving live protection. These four programs collectively use less memory than the bloatware that is Norton. Spybot - TeaTimer
A resident shield that you can be optionally installed with SpyBot - Search and Destroy. TeaTimer will protect you from registry changes, allowing you to allow or deny them as required.
ZoneAlarm
One of the best, and easiest to use, firewalls available. Will prompt you whenever something tries to access the internet, giving you the option to allow or deny it. Do not use Windows' own inbuilt firewall; it has more holes in it than Swiss cheeese.
AVG AntiVirus
A multi-award winning and excellent anti-virus system. The free edition has all the features and functions you will need, including regular updates and complete email scanning of both incoming and outgoing emails.
Spyware Blaster
Maintains a blacklist of sites known to install spy/malware and blocks such installation. Keep it updated regularly, especially if you use Internet Explorer.
This is all you really need to run. Be very careful about trying to add too many resident shields or other protection; it's impractical and may actually reduce your security as some programs may conflict with one another.
On my own PC, I have four programs in my taskbar: TeaTimer, Zonealarm, AVG and my sound card's volume control. That's it.
Having fewer programs will vastly help with speed and stability of your system. I've repaired a lot of computers in my time, and seen some truly hideous examples with a dozen or more programs running at any time.
If you have bought, rather than built, your computer, you've like had Norton or McAfee bundled onto it whether you want them or not. Uninstall them. Trust me, get rid of them; the programs above will more than replace them, and at a fraction of the memory and CPU usage.
Take a look at your taskbar right now. You should have an anti-virus and a firewall as standard, but you should be asking yourself if you really need any of the programs you have in addition to that.
Run Regularly These should be run regularly. I personally run them once a week, but at least once a month should be fine for most. CCleaner
Will help to clean out large amounts of 'junk' (benign files/cookies/etc.) that your PC picks up simply by being run. Will also cleanup the registry, all of which will help your PC run faster.
CleanUp!
Compliments the above program. This also cleans out your PC, and, in combination with CCleaner, will pretty much completely clean all your 'junk'.
Spybot - Search and Destroy
Will clean out almost all forms of spyware and malware from your system. Simple to use, powerful, and regularly updated.
AVG AntiSpyware
An excellent spyware/malware remover from the same team that produced AVG AntiVirus. In combination with Spybot, this will completely clean your system.
Optional Extras These are programs I personally choose to use, but are not absolutely necessary. Startup CPL
Ever wonder exactly what it is that gets fired up when you start your PC? This program will show you everything, then allow you to toggle them with a simple mouse click.
XP AntiSpy
Allows you to switch off the aspects of Windows XP that Micro$oft don't want you switching off. This turns XP back into something resembling a real operating system.
Tweak UI
The only thing more surprising than the sheer amount of control over your own system this program gives you, is that it is made by Micro$oft. Allows you to tailor your system the way you want it.
Hijack This!
Included here for the sake of completeness, this extremely powerful program will allow you to remove keys and links that
may be spyware at a root level. Many legitimate links - who happen to use the same technology tricks as spyware - also get detected, so this should only be used for those who are experienced with the Windows OS. You have been warned!
Safer Browsing The best way to browse the internet.
This one really is a no-brainer; use
FireFox. It is superior to IE in every way, and infinitely more secure. You are less likely to get spyware in the first place if you browse with FireFox, especially with the
NoScript plugin which will block all scripts on visited websites, then offer you the option of allowing them or not.
If you must use Internet Explorer - and there is no reason to do so - then make sure you use Spybot's IE Resident Shield.
computer spyware 
lol!
