It sounds like the resolution was reset to something lower that what you were playing in. Start the configuration program again, and increase the resolution: click Start, Programs, Larian Studios, Beyond Divinity, Configuration.
Alternately, in Windows Explorer you can browse to the folder 'C:\Program Files\Larian Studios\Beyond Divinity' and doubleclick the configtool.exe program file.
Getting a bit more advanced than 'computers for dummies', in the Beyond Divinity folder you can also edit the config.div file in Notepad (or any other text editor), and change the resolution something else. The game should work fine at other than supported resolutions (I played at 1152x864, for example), though opening the automap at higher than supported resolutions may cause crashing, depending on the system and how much higher it is.
