You can do it two ways:
1) By installing as a dual boot system so you have the choice of booting into either Linux or Windows. This ifs not strictly speaking increasing from windows itself though.
2) By installing in a virtual machine (VM) inside Windows. For example install Virtualbox in Windows and then install Linux as a guest OS under Virtualbox. That way you boot into Windows and then start Linux inside it.
Edit: There's a third way too. By installing Cygwin under Windows you can run Linux shell commands under it.
I'm afraid your dad won.