Windows Movie Maker has a brightness effect, among others. Add the clip to the timeline, and on step 2 click on "view video efects" and drag the effect to the timeline clip.
AVIs are big, so maybe try to save it as a WMV before applying effects, and then re-import it (as a WMV). If it's still too dark, maybe try to apply brightness again (effects can be applied more than once by dragging/dropping them multple times).