How can I run Windows applications on a MacBook Pro? I have a friend that wants to buy a Mac Book Pro and he has some windows applications that he needs to run. He doesn't want to buy the applications again.

Can he run the Windows applications on his MacBook Pro? What ways to accomplish this are available?
You have three choices. They all come with pros and cons

Run a Virtual Machine

You have VMWare Fusion and Parallels Desktop in the commercial space to chose from when it comes to virtualization software. And if you're comfortable tweaking and fiddling you can use VirtualBox for free.

Pros:


Near native speeds for your applications
Your OS X-based data can be accessed from your Windows programs
Can pick between running them in a full Windows desktop-in-a-Window or in integrated mode where the Window apps appear as windowed apps right in your OS X desktop
Decent support in the commercial offerings at least for graphics acceleration so you can run some graphics intensive programs
You get a bunch of neat things with virtual machines including the ability to pause applications mid-run and snapshot the state of your virtual machine 


Cons:


You have to buy a copy of the Windows operating system
The Windows OS installation will take up a lot of space on your hard drive, plus the Windows application space
If your Mac is older, doesn't have much RAM, the VM approach can drag down your system a bit but most, if not all, new Macs can handle VM hosting duties without much of any issue


Run Wine

Wine is a translation layer for Windows applications. It's a bit like a virtual machine but doesn't require the Windows OS. It sits between the Windows application and OS X and makes the application think it's running on Windows by translating all the Windows calls its making to OS X calls.

There are two choices of know of here. The free WineBottler and the paid-for CrossOver Mac from CodeWeavers.

Some commercial software is delivered for OS X this way -- I know The Sims 3 game that my wife likes to play on her MacBook Pro is actually the Windows version of the game running under the Wine emulator.

Pros:


You don't need to buy a copy of Windows
Your OS X-based data can be accessed from your Window programs


Cons:


Application support under Wine can be hit-and-miss. Some apps work well, some don't. Check at http://appdb.winehq.org/ to see if your applications are on the list of tested, supported applications if you're going to try WineBottler. CodeWeavers keeps their own list.
Because the Windows applications think they're running on Windows, and not everything Windows does translates perfectly to OS X, some applications can behave erratically.


Run Windows Native with BootCamp

Windows can actually be installed your Apple hardware directly and usually runs just fine. Apple even provides a utility called BootCamp that lets you keep a Windows and OS X installation available to your Mac and pick which OS to boot in to when you start up your machine.

Pros:


That's the fastest you can get for Windows apps on Apple hardware. It's the OS running on bare metal. If you need every last little bit of performance for your applications, this will be your best bet.
Your Windows partition stays separate from your OS X partition so changes to one rarely impact the other


Cons:


You have to buy a copy of the Windows operating system
Moving data between the Windows side to the OS X side is hard and in some cases impossible, for the most part the OS'es and their programs are completely silo'ed