Each copy of Adobe InDesign CS3 comes with a serial number. You use this serial number to activate the program---if you don't have the serial number, you can't use InDesign. This also means that if, at any point, you lose your serial number, you won't be able to uninstall and reinstall the program. However, thanks to a quirk in the activation wizard, you can simply deactivate and reactivate your copy of Adobe InDesign CS3 in order to find your serial number.
Launch Adobe InDesign CS3.
Remove the check mark from the "Erase my serial number from this computer after deactivation." Make sure this box is not checked.
Wait for the program to finish deactivating, then click "Exit."
Launch Adobe InDesign CS3. When the installation wizard appears, you'll see your CS3 serial number in the "Serial Number" field. Copy the serial number down for later use.
If you leave the "Erase my serial number from this computer after deactivation" box checked you will lose your serial number.