Your computer will sometimes not respond to a click, opening an email, clicking a new tab in explorer, and it just seems to stall. You have found that by bringing up task manager, Ctrl Alt Delete, it always breaks a stall and an action happens. There is never a CPU hog shown in task manager.
You are running Windows XP with service pack 3. You have a hardware firewall and always up to date on Microsoft updates. You have used many tools, but never discovered a virus or trojan and running Avast anti virus.
Previously you used Microsoft security essentials. Any ideas on what may be causing this? You have run online scans, anti malware tools and none of them found any virus(s) or trojan(s).
If you haven't already I recommend running a MalwareBytes scan. Also, if you haven't tried using CCleaner, to run both a PC cleaning and registry scan. Make sure to update both Malwarebytes and CCleaner first before running scans.
I have seen computers improve tremendously running both these applications. If you don't have any malware on your computer then at least Ccleaner will clean up temp files, cache, and old registry and files and folders from programs uninstalled. Another possibility is your Windows XP profile that you are logged into.
Cleaning out your temp files in your profile as well as Windows operating system temp files might help. If it's a problem with your Windows profile becoming corrupt, you can create a new Windows test profile and then log in with that profile. Test to see if you see some same performance issues with a test profile.
If you no longer see those problems in a test profile then most likely your older Windows profile has become corrupt or needs some maintenance performed on it. Delete all temp files in both temp and temporary internet files folders in your profile. You can run an ECHO %TEMP% at a command prompt to find out a location of your profile temp folder.
It should be similar to C:\Documents and Settings\\Local Settings\Temp and C:\Documents and Settings\\Local Settings\Temporary Internet Files. Remove any unnecessary programs from starting up or running in background when you login. You can go to "Start" then select "All Programs".
Choose "Startup" and right click on any programs listed and select "Delete" to remove them from starting. Also, you can use MSCONFIG utility to disable any processes or programs from starting up when Windows boots. If you haven't used MSCONFIG before then you need to use it with caution as you can cause bigger problems if you accidentally disable a service or settings with that utility.
Another utility you can download and install is Microsoft's user profile hive cleanup service. UPHClean can improve Windows XP shutdown performance by cleaning up your profile before shutdown.