Linux

you don't need "everything".. and if you did, you'd perhaps also need to recreate the entire working windows environment. so make full hdd backup image to satisfy that requirement---or, don't reformat and reuse the drive, just pull it as-is for safekeeping--executing a full shutdown first: shutdown /s /t 0

as far as the user files go... this is the basics of what i do (migrations for home users is about half of my workload):

copy user libraries (the default locations windows saves your files to) for each user account:

robocopy c:\users(windowsuserprofiledir) d:(destinationonexternal) /e /dcopy:t /copy:dt /xjd /xa:sh /xd appdata /r:1 /w:1 /mt:2

put each user's files into their own destination directory.

use care when backing up directories linked to or taken-over by cloud services (looking at you, onedrive). make sure the files copied actually exist on the destination drive.

export bookmarks and saved passwords from every browser and browser profile from each windows user account.

backup steam or its directories for later restoration, if wanted.

save mailstores from local mail clients. not many home users run a mail client with local stores anymore. the windows-only free-to-use (but not foss) 'mailstore home' in portable mode run off an external might be useful (yes, it can run in a vm later if needed). don't forget to jot down mail server configs.

check 'public' and other places for stray files.

optionally, backup browser profile directories, zip 'em up (fastest compression is enough). if you restore entire profiles, you might preserve more of the browser environment.. but chromium ones will still choke on logins, so having password exports is critically important. most people are fine with just having bookmarks and passwords migrated--which are easily backed up, and normally what i restore instead of entire profiles (unless it's firefox to firefox, and windows to windows).

make sure you know the credentials for and can login to important sites and services from a different pc or a private window without leaning on saved sessions in existing browsers on the current windows pc.

if there's something else specifically that you want to save, like a wallpaper that you don't have the original source file for.. a quick web search will likely reveal its location.

Buy a new hard drive/SSD for your Linux installation. Put your Windows drive away in a drawer so all of its contents are saved, and you can swap it back in if you have to. A USB adapter can be helpful for retrieving files.

1. C:/Users > all user data from all users on the PC
   or just from a single user
   %userprofile%/AppData
   %userprofile%/documents
   %userprofile%/(everything else like pictures, videos, etc.)
2. C:/ProgramData > some programs saving files here, so all users can use the data
3. C:/Program Files > majority of programs are installed here
4. C:/Program Files (86x) > some programs are installed here

Exactly! Your user data is stored in c:\users. This includes, well, your user data for all of the users, including all user-spefific configuration files and application data and actual files and directories created by the user.

Unfortunately lots of configuration is stored in the registry and is useless for transitioning them over to Linux. Same with most Windows software that doesn't use the registry. You'll unfortunately also find configuration files all.over the place. Might it be in the application's installation directory c:\ProgramData, or somewhere else.

%appdata% has a lot of it. Also while you're there go up a folder, and have more folders with config and data files specific to either windows and/or programs and games.

In such cases I usually do a disk image copy after I remove apps that aren't required. That way I have all the files and I can boot it in a virtual machine if necessary.

Best option is a new seperate drive of course.

But besides that I can recommend making a image of the drive ans storing it somewhere. This way you can flash everything back, there would be a ton of applications that would not run if you try and restore the files only.

Last option should be to customly copy files from folders around. This would not guarantee you can restore Windows. But you maybe would be able to restore config at a much lower storage amount

Mainly the users folder(s) e.g. c:\users\YOURUSERNAME , the hidden appdata\local and appdata\roaming folders in there probably contain way more than you actually need to back up but you could back up the whole thing to be on the safe side. Most of your user's program configuration data is in those folders.

Sometimes systemwide config data is in the hidden c:\programdata folder but I wouldn't back that up aside from specific programs you really want to save config info for.

Aside from that any other folders you created containing data you care about.

And like the other comment mentioned, the Windows registry also has lots of program config data but I usually skip that, the majority of it is useless.. but if there's a great need for you to export a specific registry tree you could do that via the command prompt to export to a backup text file. I think reg export would do it https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/reg-export

You can use some program like macrium or whatever to make a complete compressed image of your windows drive on some other device and then pick through it when you’re ready.

That requires that you have some device with as much storage as is used on your windows drive(s).

Dd if=your_windows_drive | tar lvqf some_output_file.tar.bz

Resize your Windows partition to be smaller, then install Linux in the newly-freed space. You can boot into GParted to resize the partition.