You need to do it by phone most likely, here read below, this is a good guide i found.
" Try Internet Activation First
If you have an internet connection when activating Windows, attempt to use the online route first. In the Windows Product Activation Wizard, choose “Yes, lets activate Windows over the internet now”, followed by “No, do not register with Microsoft”. You may receive a friendly “Thank you!” and your copy of Windows will be good to go.
If internet activation fails and the Wizard complains about “too many activations” or an “incorrect product key”, you still have the option of calling Microsoft’s toll free number to activate Windows. As long as the Wizard doesn’t report “invalid product key,” options are still open. An “invalid” (not “incorrect,” mind you) key is one reported or detected as pirated, so you’ll have to find some other way to make your software go.Phone Activation Phone Activation Phone Activation Phone Activation
Activating Windows via phone is about as enjoyable as a trip to the dentist, but knowing exactly which buttons to push and the right words to say can help cut your time in half and get you the necessary code.
If your online activation from above failed, click the Telephone button to generate an Installation ID, which you’ll read off to a customer service representative shortly. Call the number for your appropriate country, and you’ll enter into a phone menu system which would like nothing more than for you to activate by reading the numbers onscreen. The software behind the menu, however, does the same checking as the online activation method — meaning if your online activation failed, so too will the automated phone activation. Press 0 on the phone keypad when asked if you are calling to activate Windows XP (the first opportunity you are prompted for input). Like many newer phone menus, it will respond “I see that you’d like to transfer to a customer service representative”, and subsequently try to convince you to go back to the automated system. Ignore the pre-recorded woman’s somewhat rude request to go back, and press 1 to to talk to a human.
After a short delay and the distinct possibility of listening to some poorly chosen music, a representative will ask for either the first six digits of your Installation ID, or the full set. (The first six digits of the Installation ID are the only real keys to generating a Confirmation ID, however the reps more often than not ask you to read off the rest of the group. The rest of the numbers are encoded values detailing some very basic hardware details about your computer.)
If asked “Is this the first time you’re activating Windows?”, say yes. If you say no, the representative will ask for an explanation as to why you’re re-activating Windows.
If asked “Did you purchase the software from a retail store?”, say “It came pre-installed with my computer.”
As a final time-kill (their system sometimes takes a minute to generate Confirmation IDs on days with a particularly heavy load), you may be asked for your Product Key (Windows serial number).
After you’ve given them the necessary information, the representative will read the Confirmation ID to you, which you can simply type into the provided fields. Using these methods, it takes about 4 minutes to activate Windows on average. "
Source: How to Activate Windows XP
Goto that website for more detail.