1) Scan your computer with security applications; antivirus, antispyware/antimalware and
2) disable the startup programs in Start button\Run\type; "msconfig" (no quotes) and press Enter or click ok... then in the Startup tab, uncheck all processes except for the antivirus process/es.
3) Next restore the default security permissions using SECEDIT.EXE (for Windows XP Professional) or Using SubInACL for Windows XP Home Edition (and Professional Edition).
4) You may also have to disable the antivirus right before the installation because they can interfere with the installation..
5) If you're installing the service pack from Windows Update, better download it from the Microsoft Download Center... and better make it SP3 for best protection (the SP1 package may not be available for download anyway) so it's better to install the Service Pack from the hard drive or burned CD.
6) Once you're ready to install the Service Pack, disconnect the computer from the internet unplugging the wire or turning off the connection device and disable the antivirus unchecking it in msconfig, and next right click on the antivirus Icon in the notification area of the taskbar, and if the option is available, select disable... if not, search for how to disable it in your antivirus website.