Convert Video from .avi to iTunes format keeping quality

[LERsince1991]

What program is there that I can use to convert alot of .avi video files to an iTunes (and xbox 360) freindly format, without loosing the quality of the .avi files?

Also what program could I use to onvert my DVD collection to the same format without lose of quality?

I will be converting on a PC so the programs will need to be compatible with Vista

For information on what formats the xbox 360 supports: LINK

Thanks in advance

 

[LERsince1991]

I would also like to sync the files to my ipod when i want without converting the files again.

So in effect I need a program and a configuration that can convert my .avi video files into a universal format configuration that can be used on:
--> Xbox 360
--> Apple TV
--> iPod classic (latest model)
--> iTunes (on laptops)

and with the least lose of quality (mainly resolution I'm bothered about)
Can anyone help please?