Well, not knowing how big your harddrive is, and if you've already run disk cleanup to get rid of all the unneeded system files, I find usually the biggest culprit for harddrive consumption is media files (movies/music/photos/etc) (which you can confirm via the opensource WinDirStat tool). The quickest way to free up space would probably be to grab a usb or external harddrive and just bulk push the files you don't need regularly there (particularly movies) and remove them from your computer. The more "spring cleaning" approach, would be something like:
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Music: start by deleting all the "broken-hearted/disliked" songs from your collection. Then, if data-usage isn't a concern and you're subscribed to a music service, remove all the other songs except for a minimal set of playlists that you want to have access to when you're connection-less, and just stream the rest.
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Movies: Delete the movies you don't think you'll watch again. Much like for music, if you're subscribed to a video service, remove all the movies except for your absolute favorites that you want on hand, and then just redownload or stream the others when you feel the urge to watch it again, otherwise just move them to your usb/external harddrive
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Photos: I generally recommend going through your pictures at least a couple of times a year (if not more) and a) if you take lots of duplicate photos (finding the right angle, etc) go through them and actually delete all the extras except for the top contenders and b) picking out the top pictures/albums and move them to the cloud so that they're safe in case anything happens to your computer (this has saved me many times - bonus, you can now access them on any device to show ppl). The rest can be "packed away" to your external harddrive/usb, since you've already decided that they didn't make the cut.
Emails can also be a really big space consumer, which is why many email clients offer an "archiving" service to free up space (but that's only a problem to consider if you get tons of emails)
Anyway, hope that helps
- FWIW, even if you don't end up installing the tech preview, it's probably good to free up the space on your harddrive, since (without getting into too many details of how it works) windows uses the extra space to help with performance, so you should see your computer speed up a bit
Cheers,
J