For troubleshooting Windows OS problems like performance slow downs it can be useful to try starting the system in safe mode especially if you have already tried doing virus and spyware scans.
Using the F8 Method
1. Restart your computer.
2. When the machine first starts again it will generally list some equipment that is installed in your machine, amount of memory, hard drives installed etc. At this point you should gently tap the F8 key repeatedly until you are presented with a Windows XP Advanced Options menu.
3. Select the option for Safe Mode using the arrow keys.
4. Then press enter on your keyboard to boot into Safe Mode.
5. Do whatever tasks you require and when you are done reboot to boot back into normal mode.
When using the arrow keys to select the startup mode either SAFE MODE or SAFE MODE WITH NETWORKING could be appropriate.
The following Microsoft support link has further details:
Microsoft Windows failsafe boot
If your computer works better/faster in safe mode then one of the startup programs could be causing the problems.
Michael Lin’s Startup control panel can be used to selectively control which programs load at startup. Finding the culprit could take some trial and error. Try turning programs on one at a time until the problem reoccurs to identify the problem program that you need to remove or replace.
If your computer still doesn’t run well in safe mode then it may be time for reinstalling the operating system from your original system disk or image if available.