I purchased a Compaq PC in 2005 that had Windows XP with service pack 2 already loaded. Sometimes the computer got very slow, as it has "integrated graphics" that gobbles up as much as 128 MB of memory, leaving as little as 384 MB of memory, so I removed the 512 mb it came with, and installed 2 GB and my computer's response time got much better.
The anti-virus programs have to do more work due to all the malware out there, your current memory just might not be enough for it to run and all your other programs to run smoothly also. The extreme slowness might be caused by excessive paging (using the hard drive for virtual memory).
So, perhaps you do need more memory.
I installed service pack 3 a little earlier this year and have had no problems.
Memory is going bad could be a cause of your problems. Be aware that your motherboard is five years old, and for some computers, that is darn old, they don't last that long.
You can use a memory diagnostic program that does not require Windows to operate - memtest86+
This program is available on the Ubuntu CD, just open the drive, set the CD in, and shut down Windows and then power up and be sure to select the CD drive to boot from.
The program is created by :
http://memtest.org/
You can download the ISO image from this link :
http://memtest.org/#downiso
and burn it to a CD (using the burn-ISO ability most CD software has today)
Let the software run for several hours as it makes a full pass using many different bit patterns, you are looking for a failure.
A failure could mean the memory is failing or a chip on the motherboard is failing.
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So I went to the Asus website, and that is a very badly designed website. I had to struggle to get to this link below, I think they make so many products they are trying to cover all bases - but where is simple information?
At this link Asus wants to install a "Asus Navigation Wizard" which I suppose allows them to interrogate the motherboard and then look it up in the database :
But if you can't run the computer reliably, it doesn't do you much good.
http://support.asus.com/download/download.aspx?SLanguage=en-us
Others have suggested re-installing Windows XP - which will require you to reinstall all your software that has been installed over time, etc. I suggest you copy all your important data over to an external hard drive now, for if it becomes necessary to reinstall XP you will lose all your data.
Another option is to remove the current hard drive, then get another hard drive and install it into the computer, then install Windows XP and the service packs and your anti-virus software - all without connecting to the Internet. If the computer works reliably, you know the hardware is good and maybe you have software corruption - which could be caused by a rootkit - and rootkits are usually extremely difficult to remove.