I buy a laptop "dell" comes with Windows installation CD (home edition with SP2), i did formating to the hard disk and install Windows from this CD, it  did not ask me any serial number or PRODUCT key, but it have some simple problems so i REPAIR it using the same installation CD. and enter the product key on lable  on my laptop when windows ask that through installation, and windows accept this key
now he ask me the activation code within only 30 days, so i follow the steps of activation and chose register and activatie windows at the same time.
after that he ask me the product key, in fact "New Key", i enter the same number code on the lable, be he refuse that and ask the key or "new key" what can i do to solve this problem?
Choose to activate by Internet, not by phone.And this is the original CD that came with this Dell? And it was purchased directly from Dell by you?

If the answer to both of these is not yes, then you have a problem.

How about some details about the CD and the computer?Yes, i chose activation by internet not by phone, but he asked the new product key..
This CD is came with the laptop (original)..
Try it without registering.when you do an internet activation on the screen should be a set of numbers 7 groups of 6 numbers.
these are the activation codes not the key on the disc or the sticker.

the numbers on the sticker or on the laptop are the product key.

for EXAMPLE Product Key equals:- rcfqd p9dx9 46wvm k2cd4 d435m

for example activation key:-123435 066960 482140 773430 635415 883054 726554

hoppe this is helpfull.The rquired number is product key not the activation key, consist of 25 digits with 5 groups ..
and found on the stickers on the laptop.. as shown in the pic attached.,
i enter the same number that i used through installation, but he said it is incorrectI'm afraid fighting with MS is the only solution here....

But be prersistent and you will do fine,I agree with "patio" , it can be done only it takes patience and persistance, dont get angry or annoyed by them they are only doing a scripted job and go by the rules. i have always managed to get them to reconsider if you have a valid legal copy you should win.check your keys. use a magnifying glass to make sure that the "0"zero's and letter O's are inserted correctly, that was always a problem.  
and check with microsoft persistance and patience, dont get angry with them they are only doing a job, they are REALLY trying to help but if you get abusive you have had it.  check your keys. use a magnifying glass to make sure that the "0"zero's and letter O's are inserted correctly, that was always a problem.  
and check with microsoft persistance and patience, dont get angry with them they are only doing a job, they are really trying to help but if you get abusive you have had it.Do they even mix 0's and o's? I believe they just use 0's.

If you phone Microsoft to activate your code and then fill out the wrong code, you'll get one of their employees on the line. Tell him exactly what happened and he'll most likely give you a new code.It's probably a CD mismatch.  You can't use an OEM key to activate a retail CD and vice versa.  Dell may have given you the wrong CD.