Assuming we are talking Windows XP here, and also assuming it is just the screen resolution that has been inadvertently changed, do this...
Right click on the Desktop, left click Properties and then click on the last Tab on the right....Settings
Slide the Screen Resolution slider control up to at least 1024 x 768 (or even higher if on a wide flat screen).
If it's Vista I recall that when you right click on the Desktop area you choose Personalization, then Display Settings, then Resolution in order to make the change.