I presume this is a CRT monitor, not an LCD one??
Either you need to adjust the "barrel" distortion setting - or, if it's pretty bad and a sudden change from OK, then it's a monitor fault. In a CRT it's probably not worth even trying a repair (which is likely to be difficult to arrange, and not cheap). A new LCD screen is not so expensive and you'll have a three year warranty on most if not all makes.