2. A virtual machine like Parallels and VMWare Fusion allow you to run windows at the same time you are running OS X.
VMWare fusion was better than Parallels -
As of 2010 Parallels is getting more recommendations
See theappleblog.com/2008/11/11/vmware-fusion-2-vs-parallels-desktop-4-lets-dance/

In either case you need to purchase a copy of Windows.
See Win 7 versions for notes on OEM vs Full version.

At Performance: Boot Camp VM vs Imported VM? - Parallels Forums they say,
I am running my BootCamp Win7 sometimes natively via BootCamp and sometimes via Parallels. But booting it up in Parallels it really takes a loooooooong time to boot up (more than 5 Minutes until I am able to work). Starting a program takes a very long time too.

So I set up a new Win7 in a VM using a virtual HDD. This one feels way faster and also boots up way faster. But I am not finished yet installing the same programs etc. to have a compare - able system for testing.

Bootcamp vs. Parallels - NewTek Discussions sys, "The main disadvantage of virtualization is of course speed. even if you let the emulation use all cores of your mac, it is about 20-40% slower than a native running windows environment on the same machine."
Its possible to see the windows disk while in OSX, the OSX partition is invisible to windows in bootcamp. (That may not be true with ver 3)

Run Applications/utilities/Boot Camp Assistant
  Don't check "Download WIndows Support Software" you will just get an error message.
  Instead insert your Mac OS X Install DVD
Drag the slider to set the disk partition size:
  Windows 7 20 GB
  Applications  10 GB
  Spare         10 GB
  Total w/o games 40 GB
  Games         20 GB
  Total w/ games  60 GB

I tried again and it said there was no boot disk.
It booted on the 3rd try.

Insert Snow Leopard dvd and run setup.exe
boot Camp x64 is unsupported on this computer

mac forum says you need bootcamp 3.2
When you try to install 3.2 it says 3.1 is required first
even after downloading and installing 3.1 I get the same message

Using:
Boot Camp: MacBook Pro built-in keyboard mapping in Windows

Parallels:

Keyboard map (Set with Keyboard in Preferences)
Mac          Windows
Cmd            Win 
Ctr+Alt+Insert Ctrl+Alt+Delete

                  Me         Others: MBP 2.2GHz 8Gb | iMac 4QC 4x4Gb 
              Parallels  Bootcamp       parallels    Bootcamp
    Monitors 1      2     1    2                    
 Processor   4.5   4.5    6.0  6.0        5.5          7.6
 Memory      4.5   4.5    5.9  5.9        7.7          7.6
 Graphics    4.9   4.9    5.2  5.0        5.9          7.6
 Gaming      4.4   4.4    5.5  5.5        5.6          7.6
 Primary HD  6.9   6.9    5.3  5.3        6.0          6.9
 
 The people with more memory were getting better performance so I increased the memory allocated to parallels from 1GB to 1.5 GB but it didn't make any difference.