Well, you're question wasn't exactly accurate.. like for example how do we know if you have Windows XP, Vista OR Mac. But anways if you have Xp/Vista go to Start>Control Pannel and it should be around there somewhere. Similar with mac Apple>System Preferences..
Can you see the screen saver when you go back to your PC?? If not, it could be your settings for powersaving where your monitor may turn off after a certain duration. This can be adjusted if this is the case.
Your computer needs a defrag to sort its files out it like a libary u use folders defrag puts them away in order in memory and while you computer is hibernating eg. Screensaver makes it even slower takes few sec usally it normal
