Press the right Mouse button anywhere in an empty bit of desktop and click on Properties.
On the desktop properties check on the screen saver part that you have not set a short time, like 30 seconds etc.
Also chaeck in the start-up folder (Start, programs, start up) that you have not got a screen saver type program running in there.
Lastly and severly techy is this for windows 98/me
Goto start, then run, type MSconfig.exe [enter]
goto the start-up section and just check what is running when windows starts up, you might get a surprise!