The first alternative is a dual-boot arrangement between OS X and Windows provided with OS X.
Windows on Intel Macs
There are presently several alternatives for running Windows on Intel Macs.
1. Install the Apple Boot Camp software. Purchase Windows
XP w/Service Pak2, Vista, or Windows 7. For Boot Camp
4.0 and above you can only use Windows 7 or later. Follow
instructions in the Boot Camp documentation on
installation of Boot Camp, creating Driver CD, and
installing Windows. Boot Camp enables you to boot the
computer into OS X or Windows.
2. Parallels Desktop for Mac and Windows XP, Vista Business,
Vista Ultimate, or Windows 7. Parallels is software
virtualization that enables running Windows concurrently
with OS X.
3. VM Fusion and Windows XP, Vista Business, Vista Ultimate,
or Windows 7. VM Fusion is software virtualization that
enables running Windows concurrently with OS X.
4. CrossOver which enables running many Windows
applications without having to install Windows. The
Windows applications can run concurrently with OS X.
5. VirtualBox is an Open Source freeware virtual machine such
as VM Fusion and Parallels that was developed by Solaris.
It is not as fully developed for the Mac as Parallels and VM
Note that VirtualBox, Parallels, and VM Fusion can also run other operating systems such as Linux, Unix, OS/2, Solaris, etc. There are performance differences between dual-boot systems and virtualization. The latter tend to be a little slower (not much) and do not provide the video performance of the dual-boot system. See MacTech Labs- Virtualization Benchmarks, January 2013 | MacTech for comparisons of Boot Camp, Parallels, and VM Fusion. Boot Camp is only available with Leopard or Snow Leopard. Except for Crossover and a couple of similar alternatives like DarWine you must have a valid installer disc for Windows.
You must also have an internal optical drive for installing Windows. Windows cannot be installed from an external optical drive.