My advice to those who don’t want to pay for Windows would be to install a GNU/Linux system. If you add WINE on top of said OS, you can run a large number of Windows programs. For those programs you can’t run, I would recommend using a virtual environment as Goretsky already pointed out.
There is also ReactOS, a binary-compatible clone of Windows. However, ReactOS is currently in Alpha status, and therefore should not be considered stable.