Sleepy Windows

Using XP SP3
After a short period of inactivity my XP seems to go asleep. It doesn't
respond to any mice clicking. I usualy have to press Alt-Tab to awake it
before I may continue to work or whatever else I want. I guess it isn't
proper. Why is it happening? What can I do to settle it?
Thanks
Kuzman Novak
Belgrade
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