Why does this old PC game keep closing?

You might want to look into a program called BGTutu, which was originally intended to allow you to play BG1 and BG2 in a single continuous playthrough using the same engine. It applies some graphical fixes, including some updates for modern video cards. The software requires both the first and second game, but the same set of developers have produce community fixpacks available from their site which fix some of the problems the game encounters on modern hardware configurations. Alternatively, you might try updating your video drivers and installing the most recent developer supplied update for the game (or installing the Throne of Bhaal expansion, which applies its own update).