The Computergeist, actually a Matrix Demon, is a ghost who possessed the Ecto-X.
Egon mistakenly thought Matrix Demon was a Computergeist, and the demon possessed Ecto-X's memory circuits and trapped Egon in a Ghost Trap. The demon later trapped the other Ghostbusters and then attempted to free the ghosts in the Containment Unit. Peter Venkman communicated with Slimer telepathically and Slimer slimed the Ecto-X, short circuiting it. The Ghostbusters, now free, use blast Ecto-X with Proton Streams and the demon surrenders.