Linux

OP, please don't let the other users scare you off. I've installed Linux dozens of times on dozens of different computers and have never once lost data while doing it, not unless I explicitly choose the option installer telling it there was nothing I wanted to keep (which is labelled "DANGER - YOU WILL LOSE DATA" in red letters). Linux Mint installer has an option to let you keep your existing OS and install Linux alongside it in a "dual-boot" configuration. This means that when you install, you permanently set aside a portion of the capacity of your boot disk (hard drive, SSD etc.) for use by Linux. The total capacity of your Windows partition will shrink by that much and Linux will live in a new partition in that space (e.g. if you have a 1TB SSD and set aside 250GB for Linux, from then on Windows will start seeing your C: drive as being 750GB large and Linux will have a brand new 250GB volume as its equivalent of the C drive). You can change how much space each OS has down the line, but it's really annoying and requires you to boot off a flash drive and not be able to use your computer for several hours while it rearranges its data.

After that, each time you turn your computer on, you'll be asked whether you want to boot into Windows or Linux. (This will come in very handy if Linux borks itself and you need something working to be able to Google for solutions and use your computer as a computer until you can figure out how to fix it. Or if you decide down the road that the Linux way of doing things just gets under your skin and you want to go back to how your computer was before.) While booted into Linux, you'll be able to access all the files on your Windows C: drive as though it were an external drive, but not vice versa. If you want to send files from Linux to Windows, you'll have to boot into Linux and copy them over. Note that from the perspective of any apps you install on either OS, your Windows and Linux partitions are two totally separate computers, so expect to be asked to sign in again.

All that said, having backups is never a bad idea if you can afford it. If you can't, a surefire way to keep Linux installer from erasing your Windows files is to put two SSDs in your machine, one for Windows and one for Linux, and disconnect the Windows one until you've finished installing Linux. This is what I usually do, and as a bonus gives more space for both OSes, although it's by no means necessary.