It would depend on how much quality you want to retain. Do you plan on using a particular program to encode the video?
The closest I've come to converting a .MOV video to .AVI be converting a H.264 one to Xvid. The .AVI file was larger than the original, maybe because I was trying to keep a decent level of detail, and I suspect the end result would be the same with the .MOV file. I'm assuming the .MOV file is also of the H.264 format. I'm not very familiar with .MOV files, so I could be all wrong.
Is the video large in resolution or length? If it's one of those fancy high-resolution videos, you can decrease the filesize, without the end result looking too bad, by altering the video dimensions. When uploading to Youtube, I reduce the horizontal size to about 512px, because that's good enough for me.
Maybe try this (very) old tutorial:
Is your machine still the one you said was a bit low-end? If the computadora has two cores (dual core), then you could probably convert the .MOV to .MP4 using either HandBrake or Avidemux and x264 (or MPEG4-AVC in avidemux).
I think HandBrake is easier to use.
The problem with H.264 encoding is that it could take all day on a lower-end machine. Once it took me 3 days to encode a 45 minute episode of some crime drama using a machine with 1 core running at 2GHz. What hurt my feelings most is that the end result was noticeably inferior
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If you are going to use it, stick with the default encoder settings, it's less complicated that way. AVI won't support it, so it'd have to output as an MP4 (or M4V).
Other people are better than me at this. Especially at 2am.