The correct answer in this thread Re-activating CS3 (Updated)
describes the whole procedure for getting a replacement CS3 installer, a new serial number and what steps you must follow to make it work.
IMPORTANT! You need an OS user account with full admin permissions so you can uninstall and install software.
If you have any old Creative Suite products on your computer, you need to uninstall them AND REMOVE the old Adobe PCD folder.
Post back if you have further questions.