My PC also ran slow with Norton, Not sure what AVG is?
Increasing RAM is always a good idea, it is the single thing that will make the most difference to the speed of a computer.
Zone Alarm should not effect the system speed, at first it may, because it will keep asking you if X is allowed to communicate with the outside world, once it has done this, then it will simply allow the traffic out. (You may see a very minute performance drop because zone alarm will taking a piece of you 128Mb RAM)
AVG - AntiVirus?? anti virus software will always slow a PC down, some are not as bad as others, I personally rate McAfee as a pretty good anti virus package, it does slow the system down slightly.
If you play online games, you may get considerably higher ping times when using a firewall.
If you are comparing it with a PC which has no such programs installed, then yes it will. If you are comparing it to a PC with Norton installed, then no, it will actually speed it up, possibly quite a bit!
hakro. Go to the Zone Alarm folder and look for a file called something like 'uninst.exe' with an icon that looks like a trash can. That should uninstall it.
If not, you will have to search google for the missing dll and install it to your pc.
You've had a lot of advice from the above posts. All decent AV packages can be set to low or high intervention according to your requirements. You can make the package scan everything in and out, inspect URLs, dance sing and provide the Hallelujah Chorus if you wish.
As Ox00 says, CPU power and RAM are key to the effect that AV intervention can have.
So the naswer lies in your hands. And btw, Norton is the worst for slowing stuff down and I've kicked it out in favour of McAfee (although I'm trying Kaspersky on my 64K XP system).