Windows XP: Why the machine slow for a minute when I put my machine for some time?

Solution 1:

Could be power management turning your hard drives off after 30min, try experimenting with changing those settings, like making them a short time and see if the symptoms happen sooner, and try making them longer and see if the symptoms come in later. See if the symptoms match the time set there. I can't guarantee that that causes that symptom but perhaps it does. You'd find out if you try that test though.

Solution 2:

It sounds like the system is indeed defragmenting the drive while it’s idle. Once you press a key or mouse button, it may take Windows a moment or two to stop defragging. If it bothers you or is problematic, you can turn it off.

In Windows XP, you can edit the registry or just use TweakUI:

(In Windows Vista and up, you need to delete the task or uncheck the Run on schedule option in the Defrag program.)

Solution 3:

I would like to know why it would slow when I leave the machine idle for some time (surely not hiberate)

It is because the system was idle for a while, so Windows paged out the memory of most of your programs to disk. As a result, when you start using them again, it has to page all of that back into RAM which means that for a while (a few seconds to a few minutes depending on how much memory needs to be read and the speed of the disk and location of the swapfile), it will be slow and sluggish. Once the memory is read back into RAM from disk, the system should start getting more responsive again.

What does the windows scheduler do? I guess it is some of the memory allocation problem

The Task Scheduler is exactly what its name implies: it lets you schedule programs to be run at specific times or under specific circumstances, and then runs them. It has nothing to do with your issue.