Seems like more people are running into their programs slowing down. As I mentioned in this post about building a graphics station, you add implement a 3GB switch to run with 4 GB of ram. There are also other things you need to keep in mind with software and the settings in XP. Here are a couple links that give a great write up about them.
System File Swap Settings
Implementing the 3GB Switch
Yes, the above links are targeted towards specific software, but these are principles you can apply to your machine.