VM
Running two OS on same machine
For virtual desktops the computer should have at least 2 GB RAM.
VM Windows On Windows
If you have Windows 7 and want to run Windows XP you can use Windows Virtual PC for free.
Otherwise, you will need the installation iso or disc, the Windows key, VirtualBox installed.
VirtualBox is free Open Source software.
From VirtualBox you can mount the iso or CD and begin the installation.
VM Windows On Mac
On Mac use VMware Fusion (free trial)
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Less than 2 GB RAM?
If your computer does not support 2 GB RAM you can create a dual boot installation.