Even with the DivX/Xvid CODEC's, it is likely that neither file will edit well in PE. VLC player has its own set of CODEC's, so it will play most formats and CODEC's.
Now, there is a great difference between playing an AV file and editing that same file. Having the right CODEC's (or access to them through a player like VLC) will only insure playability. Most NLE (Non Linear Editing) programs can work with but a few for editing. As John points out, DV-AVI Type II is what PE (and big-brother PrPro) want most.
You are best to convert these heavily compressed delivery format files to DV-AVI Type II w/ 48KHz 16-bit PCM/WAV Audio, BEFORE Import.
Even with the proper CODEC's installed, the most common problems with DivX/Xvid will be no Video, no Audio or horrible OOS (Out Of Sync) issues. Conversion BEFORE Import will usually solve these and allow you to edit the larger converted file. Now, the quality lost in the DivX/Xvid compression will still be gone - can't bring that back. Still, you might be able to live with the results. Only you can tell.