Is there a simple way to adjust audio volume in avi files, like using MP3gain so i can set them all the same it is a problem having to keep adjusting the volume. I have tried VirtualDub but it doesnt handle my avi files as they are encoded in XVID
VirtualDub handles XVID just fine if the necessary codecs are installed.
You may not have a VFW codec for XVID, which is the only type supported by VirtualDub.
Try for example K-Lite Codec Pack.
I'm surprised that Xvid causes you problems with VirtualDub, as it's the sound you want to change, but it's been an age since I used that program. You can do it with Avidemux however (it supports Xvid, but you should be able to just copy the video directly anyway).
A couple of things to consider:
Edit: This next bit's wrong. It was my recollection of this excellent article about volume in Windows Vista. But it applies to increasing the volume of signals that are already around the maximum volume that can be represented in their format, if I understand correctly.