There is nothing you can actually do until you get the serial number for your Kindle. Also, note that this proceedure won't work on e-books from mobipocket.com and some other sites.
You will need Python. See Python On Windows XP: 7 Minutes To “Hello World!”
If you download Igor's programs, the included readme.txt includes a test example. Open a command window (START > All Programs > Accessories > Command Prompt). In the command window:
post content modified on Amazon's request
Then register the PID at your e-book seller's web site and re-download your e-books.