reducing avi file size but keeping quality????????

Hi
does anyone know how to reduce an avi or wav file format to half the size without losing the quality of the image or to control the amount of MB used?
I have a Pinnacle Studio PCTV RAVE card and it saves in avi format and the Windows Movie Maker which saves in .wmv format. In the Pinnacle PCTV Rave it save the file in avi but it keeps the file really really large and it is taking up too much space. Does anyone know how to reduce this with out losing quality? I would like to save films on to disc as vcd and in avi format, but I can only fit about 20 minuts of film on there cos of the file size in avi.
Same for the Windows Movie Maker, when I save it it goes into .wmv (Windows Movie Video), but the video quality has dropped so much that the file size is really small and th quality has gone.

Does anyone have an direct answers without me having to search anymore?

Hope someone can help

Kind regards
head-ache