Before you can install Counter-Strike you'll need to find the serial number that came with the program. Counter-Strike 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 Counter-Strike is still installed. You might be able to find your Counter-Strike serial number if you've uninstalled the program but only if the serial number information was left in the Windows Registry.
The Counter-Strike 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 Counter-Strike 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 Counter-Strike.
The product key finder program that I've confirmed will find your Counter-Strike serial number is Magical Jelly Bean Keyfinder. 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 Counter-Strike keygen programs or other kinds of Counter-Strike 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 Counter-Strike serial number obtained through a legal purchase of the software.