One of the big causes of a slow computer can be spyware and or viruses. One program that I have used thats is free is malwarebytes. It is a free program that picks up software or spyware that anti-virus software somethings does not. It can be found at malwarebytes.org. Windows XP can be a resource hog and using the system restore can cause more problems then it's worth as it wont remove the cause of the slowdown.