Over time computers collect a variety of items which increases the time that the computer needs to scans things. In addition, programs are added, the search path is sometimes altered and there is an endless series of viruses and spyware which may play a role as well. They can all conspire to make the computer dreadfully slow

Four year old computers are highly subject to a variety of failures. First is the hard drive, next the power supply, next the fans. Often the video card will go as well. These all represent the state of the art in planned obsolescence and will fail starting at about 3 years. As the fans age their cooling ability slowly diminishes and as that goes the wear of other parts accelerates If you want to successfully extend the life of the computer, replace these parts.

If you are competent you can DIY for about $200. Next rebuild the OS, install service packs and programs and use it for another 4 years. If you are not competent, just get a new computer.

Re adding memory, this is a good idea to help speed up to a point. Most computers of your vintage will accommodate a max of about 4GB and only use about 3.5 Gb. Some will accommodate more. Windows XP has it’s own memory limitations too.