Sounds like a RAM problem. Or something is eating up your processor resources.
From another thread: Right-click the Task Bar. left click "Task Manager", on the tab "Process" under Mem Usage look for an application where figure is slowly increasing and identify the Process or Application. Indicates a "leaky application".
So how much RAM do you have? What other application (program) have you loaded recently? Have you done the Windows Update routine discussed on the thread "XP slowing down". Sounds similar.