What has yet to be mentioned is the actual shelf life and runtime life of the mechanical components. CPU's running all the time will last 3-4 years before showing signs of what can end up being permanent slowdown. Harddrives are only made to last 10 years if you're lucky (especially with the large multiplatter one's that have been coming out). At 5 years, it might be worthwhile to look into new parts or a new computer.
If you have a Pentium 4 or a Pentium D, forget about upgrading. If you have a dual core you should be able to upgrade easily. New RAM sticks could solve the problem, they could be going bad or might have dead/bad sectors. A fresh install of XP should show an increase in speed due to lower installs, processes, and services running.
If you want to build a new computer look for something that is overkill for your needs. 4 gigs of RAM is most likely more then you would ever need/use but it will allow for more sectors to be used through it's lifespan essentially giving you more bang for your buck. Larger harddrives will allow for better segmentation of the drive. A 500 watt power supply will be major overkill for your system, but it will supply more than enough power which will allow everything to run better. Last of all, a higher speed processor will show little strain thus lengthening it's overall working life.
Mind the text block above. If you end up having specific questions or any clarification feel free to pm me. Hope some of that helped some.
