I used to get this error conistently...
I was able to fix it by opening the task manager in Windows XP (ctrl alt delete), clicking the processes tab, finding the Pro Tools process (Pro Tools has to be running at the time), right clicking on it, and setting its priority to high.

I also had turned off hyperthreading on my machine. though this didn't fix the problem alone it might have been part of it. Either way I've found I have other apps that are much more efficient with it off therefore I'm leaving it off.

Hope this helps someone.

thanks,
John