Most times this occurs when you mistake a letter O for the number 0 or and 8 for a B etc. Recheck the Windows sticker on your computer and magnify it to check the 25 letters/numbers and try it again. It's called an activation key, not a product key.

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@Chrisb49 wrote:

I have just reinstalled my windows xp to my dimensions 2400 (I had to reformatt the harddrive) now windows won't accept my product id key code it says I have 30 days to registar it. what do I do?

The key is on the side of the computer.  Read ZEROS and OHs carefully, also B and 8. 

If you used your Dell OEM XP CD, it should work fine.  If you used someone else's CD, it will not.