It makes me wonder greatly how in the world you installed it last time. On the other hand, if you are holding a 100% genuine xp cd, and it's in it's original case, and the key written on it doesn't work, you might be able to send it to microsoft for a replacement. I wish there was more i could say about it, but we don't have any way of helping you get a new key, and we don't have any way of helping you bypass the key. You will probably have to call microsoft, and talk to them about buying a new key. Otherwise, if this is an original xp cd, and you have the correct packaging for it, then there literally is no reason the key wouldn't work, unless you typed it in wrong. After that, wait for the process to finish. Then enter the product key that you have.
Click the Activation Windows Online Now option. Go to the Start menu > Control Panel > Windows Activation. If it's a copy, then you should know better than to ask on a microsoft partner site. But not to worry, you can get the product key that matches your Windows 7 edition on the link here. I am assuming this is an original, so you shouldn't be having trouble reading it, so we'll rule that one out. Is it possible you accidentally grabbed the jewel case for someone else's xp cd, and they have yours? If the key you are typing in comes from the packaging of the cd you are using, then it should be working fine.
