Whenever I try to launch the game, an error message shows up saying: "An Assertion failed in F:\Torment\Source\chitin\ChDimm.cpp at line number 787.

All of the other forums point to mods being the error. However, I have no mods installed. This is my first time starting the game. Any fix for this?

Have you tried this?

http://www.gog.com/forum/planescape_torment/pst_under_linux_assertion_787_error