WinXP sometimes does not turn off it's screen

[Guest Levlg]

I have WinXP+SP2, it works stable for many days (24/7) without rebooting and

turning off. But sometimes it stops to turn on it's screen saver and does

not turn off it's screen. Repetitive inputs the settings through Control

Panel do not fix the problem. This could be fix by rebooting or sometimes

this is fixing by itself. I could not catch what is the source.

 

Prompt me please, what can be cause and how I can fix it without rebooting.

 

Thank you

Lev