My computer says that i need to update Service Pack 2 but when i try to it gives me an error message that says, "Windows XP Product Key is invalid" How can i download this package?
Omega's correct. If you bought it from a shop or an individual vendor, including eBay, report them to Microsoft. If you obtained it any other way, then buy a legit version, and you'll be able to install all the service packs you want.
I would follow magicman's advice to use the validation tool at the MS link in order to grab all of the updates/security fixes. SP2 also has fixes for things like USB 2.0 as well as the short wait for SP3 that will be coming along. Many programs require what is in SP2 where fooling the registry only stops the updates are ready pop up messages.