On install, XP asks you for you product key - the key you can find on your CD/in your documentation / on the back of your box in case you have an OEM version. Without a product key, you can't install. You indeed can put in any key you want here (as long as it's a valid one).

This product key is transformed (under the hood) into an installation ID number: a combination of the product key, and the hardware in your box ("hardware hash" in MicroSoft terms) (and who knows what else).

This installation ID number is used to activate Windows. If you don't activate Windows, it'll stop working, plain and simple.
The suggestion by !mitationRust is for when you're using a pirated Windows version and you're trying to upgrade to SP1 - SP1 contains a "repair" for these cracks...

Then there's registration - which isn't mandatory. If you do so, though, you'll receive a registration ID (which can be found by right-clicking My Computer, tab General, Registered to: ).

RockXP is perfectly useful for what he's asking (given that he's obviously asking for the Product Key, which can indeed be obtained using rockXP)... not sure about the intention behind his question, though