Sounds like you either have
1) Too many processes(applications) running on in your computer,
2) BIOS not up to date. Windows XP SP2 has ACPI 2.0 and BIOS were revised on some systems to accomodate this major revision,
3) Memory errors,
4) Some spyware is installed running -- Always a bad thing. If you have installed a great deal of free-ware, this can be a real possibility.
5) A conflicted registry.
6) You have two firewalls up and running simultaneously. For example, NORTON INTERNET SECURITY and WINDOWS XP Firewall.
7) If you have NORTON INTERNET SECURITY (or any other FIREWALL Utility) installed, there can be conflicts with the updates (more on this).
Your Athlon XP 2400+ is a processor with ample capability for mainstream PC applications. Your ASUS A7N8X Deluxe is a fine motherboard that is well supported with BIOS upgrades. And 1GB of memory is plenty of real estate for Windows XP SP2 -- there should not be any harddrive paging. Of course uninstalling applications no longer needed is a great way to free up workload.
Short of re-installing Windows XP, I recommend the following steps that seems to be effective at clearing up slowed down computers of my friends:
1) Uninstall applications no longer needed.
2) Download and install the latest and greatest BIOS for your motherboard. BIOS for many motherboards were revised to accommodate changes in the Advanced Configuration and Power Interface (ACPI) with SP2.
The XP help utility says:
"During Windows Setup, ACPI is installed only if all components present during Setup support power management. Some components, especially legacy components, do not support power management and can cause erratic behavior with Advanced Power Management (APM), or may prevent ACPI from being installed. Examples of such components are Industry Standard Architecture (ISA) components and an out-of-date BIOS."
3) Test your memory with Memtest86. I recommend version 3.2 since the tests were revised with more random patterns and are more likely to uncover errors. Download and create a floopy of this utility. Reboot your machine on this floopy and execute all of the tests. Should take about 45 minutes to go through one pass of the test suite. I have found that bad memory can also cause slowdowns rather than application crashes if memory errors occur. If you find some memory errors, try increasing the voltage first. If this does not work, restore the voltage to default (2.6V), then slow down the timings (CAS, RAS, CAS-to-RAS, CMD Rate from 1T to 2T). Memtest86 is an excellent tool to do this kind of tweaking.
Moving (or removing) memory modules can help you deduce which memory module is defective. If needed, you can populate one memory slot for this test.
If tweaking the memory timing is ineffectual, replace the defective memory module.
4) Install and run some of the excellent anti-spyware utilities such as Ad-aware, SpyBot Search and Destroy, and CWS Shredder. With these utilities, make sure you have the latest definitions. I like Ad-aware the most though SpyBot S&D will find stuff that gets past Ad-aware. Spyware can really rob your system.
5) Install the excellent JV16 tool and use its registry fixer and registry cleaner to straighten out the registry. There are free versions of this tool available on CNET, but the early versions (1.0 and 1.1) are actually easier to use than the more powerful later versions. Use the registry fixer first to fix errors, then the registry cleaner second to remove worthless registry entries.
6) Make sure you have one firewall running. Turnoff the Windows XP firewall if you have another one installed.
7) Uninstall and reinstall NORTON Internet Security. In the past, I have experienced slowdowns with this utility after 6 months because upgrades apparently become conflicted. Uninstall this utility and use the Windows XP firewall to see if system performance improves.
Use the search tool to find all the directories and files with "NORTON" or "SYMANTEC" to remove and rid your system of all traces. Use JV16 to cleanup the registry. Then do a reinstall of this utility. After the reinstall, use Liveupdate to obtain the updated software and definitions. Be patient, do not disturb this process. Make sure you reboot your system after each session, then use Liveupdate again and again until there are no more updates for this utility.
This procedure has cleared up some of the slowdown problems my friends have experienced with their computers.
I think these procedure will help you too