First, download the crack from
www.gamecopyworld.com
Then, you will have an archive (usually .zip or .rar extension). Download winrar from
www.rarlabs.com. Install it.
Once you have installed WinRAR, right click on the archive you have just downloaded, and select from the new menu "Extract Here". Now, the contents of the archive will be extracted.
Look around the contents of the archive you have just extracted. You should see a .exe file. Now go to the directory of the game you have installed that you want to crack. Find an exe file that matches the name of the exe file you have just extracted. Create a Backup of the exe you have found in the games install directory. You may need the backup at a later date!
Now, copy the .exe file you have just extracted to the directory in the game where an exe file with a matching name is. It will ask you to overwrite. Click yes and let the exe copy itself. Now the game should be cracked.
Some games require that you copy over multiple files. If you are having trouble, refer to the .nfo or .txt file that comes in the archive you have just extracted for further instructions.
Thats it, and if you are having trouble understanding any of this, I suggest you read "Windows XP for Dummies".