I hate to say it but Google is a good source for this information, aside from illegally obtaining a key you can also purchase keys from Microsoft and from memory quite cheaply.
We still use XP for FAT testing at work so I'm sure they still sell the keys.

Just remember to search using the correct service pack number you have installed and if you decide to go the wrong down that one way street find a version that doesn't require a key aka is already activated.