Windows comes preinstalled with the Dvorak keyboard layout by going to
Control Panel->Regional and Language Options->Languages->Details....
Windows 8 is the 1st version of Windows to support switching the keyboard layout for all open Windows. If you are attempting to switch the layout of all open windows in less then Windows 8 I advise downloading an Autohotkey script. The scripts don't need Admin rights but needs the same rights used as the other windows to work properly. Alternatively PortableKeyboardLayout I believe can work.
I use Dvorak.exe. It is an Autohotkey script that you can that allows yout to enable and disable qwerty and Dvorak between all applications by hitting the CapsLock Key. The scripts don't require administrative rights. Additionally if you hold down the right Alt key(also known as the AltGr) the home row shift to 1234567890 number keys.
Alternatively you can use a program like Keyla. Maybe even SharpKeys
Portable Keyboard Layout.
Dvorak is one of the preinstalled input sources that can be selected in System Preferences > Language & Text > Input Sources.
wget -qO- http://limeblack.github.io/dvorak/xmodmap | xmodmap -
wget -qO- http://limeblack.github.io/dvorak/xmodmap/default.sh | sh
wget -qO- http://limeblack.github.io/dvorak/xmodmap/temp.sh | sh
In most Unix based OS's there are two ways in which you can temporarily change the keyboard layout. You can change the keyboard layout in the GUI(x11) interface or in the console interface.
You can type Dvorak online at branah.com/dvorak or inputking.com.
There also have been attempts to detect the keyboard layout in the browser, so you could potentially switch the keyboard layout through the browser online.
Additionally you can get a physical doggle called QIDO, that switches between Dvorak and QWERTY manually.
Ideally you could create a Window autohotkey script that has unix sh commands appended at the bottom. It appears appending text to the bottom of an Autohtkey executable doesn't alter it's ability to run. You can then run the file in Windows by doubling clicking on the file and convert the keyboard layout to Dvorak by running something short like in unix.
wget -qO- DvorakScriptHosted.com | tail -n 40 | sh