Security Software Conflicts
Sometimes, in our zest to protect our systems and/or because we just don't know any better, we make the "mistake" of installing more than one antivirus (AV) or firewall(fw) on our computer. I say "mistake" advisedly because there are instances where more than one AV or firewall running simultaneously on a system is harmless. For the most part, however, you are risking serious harm to your system if you try to go with more than one AV or fw. You are also running the risk of disabling other programs on your machine including one or both of your AVs.
The problem with running two antiviruses stems from the way the AV sets itself up in your system. AVs you install take up their position in your computer's memory. They are for this reason sometimes called memory resident (mr). The memory resident AV occupies the same places an invading script, or virus, would use to insinuate itself into your system. It has to in order for it to work. If another AV is on your system, chances are it will see the other AV moving into these sensitive areas of your memory and attack it. The first AV will then counterattack that AV, if it's lucky, thinking it has caught a virus designed to disable AVs (they're out there!) or vice-versa. The two competing AVs will quarantine or delete each other's core files. The result is paralyzed AV(s), a system freeze, sometimes damaged memory and an inoperative (but not inoperable )PC.
Even if your two mr AVs are getting along chances are you will have trouble when you try to use one of your programs. (Memory) Resident AVs also monitor your system continuously looking for viruses and (potential) virus activity. It's called real-time protection. For example when you open your email suite and send/receive emails or when you download a file from the internet, your AV swings into action and scans the file(s) for viruses. When you start up a program, your AV checks it out before it loads into the memory. Problems occur when you try to open a program when two mr AVs are running simultaneously and they both clash at the same point, in this case the program you open, say your email suite. Both AVs collide and see each other as malignant script converging on your email program. Here too the result will likely be damage to one or both AVs and a disabled email suite or whichever program you tried to open.
Lastly, even if your two AVs are running smoothly together, and it can happen, you run the risk of exhausting your memory and power supply because AVs tend to be resource hungry. Some of them are more than 20 MBs in size. If you run out of memory and/or power, your PC is likely to crash or freeze. Believe me, folks it's no fun when your system hangs.
The same is likely to happen when you try to run two firewalls. Firewalls are designed to stealth ports and monitor activity into and out of your ports. Firewalls inspect and give or deny permission to data(programs, emails etc.) leaving and entering your PC. If your firewall detects something suspicious or unfamiliar trying to leave or enter your PC, it denies it entry or exit until or unless you grant permission for that data to enter or leave your PC. If you have two firewalls running you often wind up with a loss of internet access or system hang as the two firewalls block and deny programs simultaneously but in an uncoordinated way. That means Internet Explorer or whichever browser you're using can be blocked from getting out to the internet. When you click on the browser icon, you wind up with the dreaded "This Page Can Not Be Displayed" message. Likewise you will be unable to send/receive email as the two firewalls will have combined in a maladaptive way to block internet access to your email suite. To put it simply, you are isolated from the internet.
That's the best case scenario. The worst case scenario is that your PC freezes and crashes due to loss of power (most firewalls consume a lot of power/memory).
How To Get Out Of Trouble
OK. Let's say the worst has happened and you tried to run more than one AV and/or firewall. You're stuck with a machine that has no internet connection (if you're lucky) or has crashed or keeps crashing when you try to use it for anything other than ornamentation.
If your problem is two competing AVs, the solution is Safe Mode, to manually delete every trace of the two AVs in safe mode. The first thing you do is turn your PC off. If you can't do it through Start, Turn Off Computer. Do it manually. Hold the Power button down for a five count or until it shuts down. Wait a minute or two and turn it on again. As the PC boots up, tap F8 repeatedly until the Windows XP Advanced Options menu appears. Using the arrow keys on the keyboard, select Safe mode and then press Enter.
Go through the uninstall procedure I outline here except include the steps in the Registry which I describe as optional and reboot only after you have finished all the steps.
Your PC shut boot up normally and return you to your desktop with your PC functioning properly or at least well enough to get some help in a forum or email someone who can assist you further.
If your problem is two competing firewalls, you may not need Safe Mode but you will have to delete one, (if not both) of the firewalls using the steps outlined as above in order to re-establish internet contact or to get your system running again.
PC Tips at PC1Stop