You can't. Xvid is a delivery format and not editable. Your best option is to get the original source material, but I have the feeling you can't get the original stuff (for legal reasons, maybe?) so in that case you need to convert outside of PR to an editable format and take the inherent quality hit as a given, or better yet, forget about it altogether.
Xvid, is a highly-compressed CODEC, and is designed for delivery-only. Premiere does very poorly with editing it, or DivX, and usually will only Import the Audio.
I would suggest using a conversion program to convert those files to something else. Many can also handle resolution reduction. I use DigitalMedia Converter 2.7 (Deskshare), but they have released newer versions, that I have not upgraded to. It will handle Xvid very well, though obviously, the quality hit that has taken place can never be improved upon.