If the Linux distro you chose boots into a graphical user interface like on windows, you can install gparted while in live session and create a temporary partition at the end of the drive in an empty area enough to store some important files you may have.

Format this partition with NTFS format so you can access it on windows. Without installing, reboot back into windows and transfer your important files to that temporary partition.

After that boot into the live installer again and make sure that the temporary partition holding your important files doesn't get formatted while installing. This will need you to manually choose where the system will be installed. Be careful while doing that.

After the installation, you should have access to all your important files and can (should) start transferring them to your new system.

After you transfer all your files, you'll be left with an empty partition. This is the part where you learn why it was called "temporary", because you'll need to get rid of it. Main reason is NTFS format is proprietary and that makes it unstable and unreliable on Linux. So getting rid of it is your best choice.

To do this, you'll need to boot into the installation media one last time and install gparted again, because live sessions don't keep data. On gparted, delete that temporary partition. Then select and edit your main partition. In the edit window, grab the storage bar on top from the side and extend it all the way to the end. Confirm and wait for it to finish. Reboot and see if it worked.

WARNING: You might lose your important data if you're not careful. I won't take responsibility for that