I think you must be got same problem with me , when u use the computer and then suddenly the application that u use running slow .
Why this happened? Because other application running too In same time , example : antivirus that running in the background.
Windows xp save the priority application rule at the registry, so you can change the priority to make windows xp set more resource to foreground application.

Follow this step
1.    start – run- regedit
2.    at registry editor go to HKEY_LOCAL_MACHINE\SYSTEM\CURRENT CONTROLSET\CONTROL\PRIORITYCONTROL , at righr panel you can see item named win32priorityseparation
3.    double click to that item and change the value to 26 with base hexdecimal or 32 in base decimal, and then click ok
4.    restart you computer and then after that your windows will run with priority foreground.