I want to install another Windows 10, so I can have two. I want to use one to "test" different things (such as some programs that could contain malware, for example). If something happens to the "test" version of Windows 10 and I have to reinstall it, I have to reinstall my "main" Windows too(because I don't want to lose any data from it)? And I can put both Windows 10 on the same SSD? I don't want to use a virtual machine because my PC is pretty old and slow and the virtual machine isn't performing very well.
Possible? Sure
Good idea?
Not even a little bit. A virus or ransomware will easily chew up your good drive and OS.
Testing like this is NOT done on your main system. Or at least, not with the daily drive and OS installed.
A completely different drive, that is the only one connected while you go down this investigation path.
Install a bare WIn 10, make a known good backup image of it.
Have ONLY that drive installed when you install this OS
Have ONLY this drive connected when you do this "testing"
Do all the malware stuff you want.
When done, recover from the Image and return that drive back to the starting point of a clean fresh install.