ANd that error message is..? What file? What system? What version of AE? You need to be a lot more specific. That said, AE doesn't really deal with Xvid, DivX and similar garbage, so you might as well look for other tools in the first place.
If you're just looking to convert a file, the Adobe Media Encoder is much more suited for such a task.
If you're dead-set on using AE, you're going to have to give more information, as Mylenium says.