Windows Sp3 installation just ends halfway

[nds9]

Hi guys i got Windows xp PRO with SP2 and so i wanted to upgrade to SP3. I downloaded the package from microsoft but when i go to install it goes on half-way but when it comes to registering registry then it closes and shows a message "the specified file was not found" and then says me to click ok to roll back changes and Reboots. After restart a message appears saying "Your system has been partially updated. Some functions may not work properly". Someone help