I thought windows.old was a folder not a file?
In previous version of windows it was and installations put all old files into there so there is a fallback. If you did a clean install it saved everything there and you could recover afterwards.
If it is a folder have a look at its contents - if "my documents" and other folders are there and the contents are not in the new folders then recover them from there.
Assuming there is nothing you need in there you "can" delete this. Google "windows.old" - many hits about this.
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