If you're installing a program for the first time but don't have the product key the software asks for during installation, you probably won't be able to proceed. Some programs let you enter the product key later on after the installation is complete, but many do not.
Several websites exist that help computer users install software with no CD key. Some sites provide lists of free product keys, others link to special programs called product key generators that create unique keys for individual software programs, and still others provide programs called "no CD cracks" that allow a program to be installed without entering a product key at all.
None of these methods are legal. Each is simply a different way of pirating a software program.
Using a product key not issued by the software developer or circumventing the need for a product key altogether is a violation of the agreement you make when you use the software. Requiring a valid, unique product key is a software company's way of ensuring that each copy of their program is used only once and that each customer is paying for each copy used.
If you currently have a legal copy of a program installed but you've lost your CD key, try a free product key finder program. This is a perfectly legal way of obtaining a product key because it's the same one you purchased when you first installed the program.