Q:

New PC, New Operating System. Can I transfer my Steam games and data?

I used to use a Mac with some Steam games like CS:GO, I'm gonna build my gaming PC soon (on Windows 10). Is transferring Steam games possible to another computer with another operating system possible. And if, how? Do I just log in and install the games or is the process different?

A:

If the games have Steamplay, then yes, games that you have on your Mac will be able to be played on your Windows machine.
If they do not have Steamplay, you will have to buy the Windows version even if you already have the Mac version.
From the Steamplay Documentation:

Steam Play allows you to purchase your games once and play anywhere. Whether you have purchased your Steam Play enabled game on a Mac or PC (both Windows and Linux), you will be able to play on the other platform free of charge.

