operating system

my operating system is windows98 on a presario desktop. Recently, when i start the pc I get the message in a blank screen that “operating system is not found on any devise, press any button to try again” I have to shut the pc down and try again manytimes during the course of a day and sometimes two days, before I get the windows logo again and everything working fine. The sad thing is that this is reccuring, lately, every time I shut the pc off.