Before you can install AutoCAD you'll need to find the serial number that came with the program. AutoCAD requires that you enter a unique serial number before using it.
If you've lost your serial number, there is a way to find it as long as AutoCAD is still installed. You might be able to find your AutoCAD serial number if you've uninstalled the program but only if the serial number information was left in the Windows Registry.
The AutoCAD serial number is actually more of a product key than a serial number but as with most software, the terms are usually used interchangeably.
Assuming AutoCAD is still installed, the easiest way to find your unique serial number is by using a free product key finder program. Product key finder programs automatically search your computer for the product keys and serial numbers that your software programs store in the registry.
Most product key finder programs are designed to find the product keys for operating systems like Windows XP or Windows Vista but some of them find product keys and serial numbers for several other programs too, including AutoCAD.
Magical Jelly Bean Keyfinder is a product key finder program that should find the serial number for most AutoCAD versions, though it was reported to me that it will not find the one for AutoCAD 2007. If it doesn't find your serial number, give some of the other key finder programs a try that I linked to a few paragraphs back. Be sure to let me know that it didn't work for you so I can keep this page updated.
Important: You may have come across AutoCAD keygen programs or other kinds of AutoCAD cracks but keep in mind that these are not legal ways of obtaining serial numbers or product keys. The only legal way of installing this program is by using a valid AutoCAD serial number obtained through a legal purchase of the software.