Download & Install version 1.5.1

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The current stable version is 1.5.1 (15 May 2011).

Download the zip file from Download [processing.org]
If Java is already installed on your PC, select (without Java).
Otherwise select Windows.

It can be unzipped to any location.
A good choice wouild be, for example:
  C:\processing-2.0a4
Processing can then be executed with the command:
  C:\processing-2.0a4\processing