Whats the best way to render on Windows?! Should I finally get rid of ADOBE?

Hey, fusion is free. Try it, then come back and give your take on the differences. Proper software doesn't exist in a monopolistic void, as hard as adobe tries to clear its orbit. In the meantime, it might be worth switching to a more modern mezzanine format. AVI used to be the best of the best, but it's protty bulky, I'd know. I use it too for its transparency. I suggest one of the DNx formats. 

Oh also there's nuke NC. That's got a 1080p limit but it's what nuke is effectively based on.

Maybe the AE is exporting/rendering on the same drive that is running the system. Try working on a separete drive. Also, you can export to Mp4 using Media Encoder directly from AE too, so you don't have to deal with massive files. 

Your workflow is not making a lot of sense. Given that AVI is a dead format and even Microsoft have dropped it more or less there's only a handful of options are available, including the default uncompressed one. Rendering to ProRes, DnX, Arri, RED XDCAM or whatever alternate CoDec would be much more efficient and at the end of the day even image sequences with compressed TIFFs and EXRs consume a lot less disk space than a chunky AVI file.

Mylenium

Why do you render to AVI, then to mp4? Why not just render to mp4 through ME? Quality doesn't drop if you set your render correctly in ME. AVI files are lossless so it will be a huge file - but 300 GB? weird. Is that render 4K res?