Microsoft makes use of CD Keys or Serial Keys to prevent piracy of its products. Suites such as Microsoft Office or operating systems such as Windows 7 take years of hard work to build and stabilise, and piracy results in those involved being denied pay which they have worked hard for. Your CD Key will be unique to your specific product as it will have been generated using an algorithm in conjunction with the serial number of your copy of the Windows XP Service Pack. The only official way to obtain the CD Key and activate your product will be to look inside the packaging it came with. It will be clearly printed and you will simply need to copy it into the appropriate dialog box that you see during installation.
If you have lost your CD Key, you could try contacting Microsoft's product support and asking for a copy of the CD Key, but it is likely you will be charged a fee for it to ensure you are not pirating the software.
One final route to go down is that of piracy. It is not legal and thoroughly inadvisable, but many peer to peer networks have links to 'keygen' applications. These install on your computer and use your Serial Number to calculate your CD Key using the same algorithm that was used to make it in the first place. Beware that peer to peer networks are often under observation and you may be fined for using a keygen. It is also highly likely that the application will install some sort of malware on your computer once you have downloaded it. Remember - there's no honour among thieves!
If you have lost your CD Key, you could try contacting Microsoft's product support and asking for a copy of the CD Key, but it is likely you will be charged a fee for it to ensure you are not pirating the software.
One final route to go down is that of piracy. It is not legal and thoroughly inadvisable, but many peer to peer networks have links to 'keygen' applications. These install on your computer and use your Serial Number to calculate your CD Key using the same algorithm that was used to make it in the first place. Beware that peer to peer networks are often under observation and you may be fined for using a keygen. It is also highly likely that the application will install some sort of malware on your computer once you have downloaded it. Remember - there's no honour among thieves!