You might consider that even though you are using the same software on the same computer, that software and its updates are becoming bigger and more taxing on resources. For example, if you are using IB, you might be running a Java that is much more resource intensive than one from 3 yrs ago. The same with QT. Also, you might have a lot of data accumulated on your QT that is eating into available memory, etc.
I reformat and rebuild my trading systems 2x per year. I upgrade every 3 yrs. You can go to various websites that help you tweak your PC for best performance depending on your needs. There is a lot you can turn off in XP that can improve performance slightly.
The hardware upgrades suggested by others are also an important step to stay up to date. The life of a computer is really no more than 3 yrs or so if you want to maintain performance.