If you are running Windows XP Professional and it is not yet up to date at Service Pack 2 (SP2), please go to Windows Update and install all available critical and important updates, including Service Pack 2 which will be offered to you. Continue to visit Windows Update until all updates have been offered and only optional updates remain after a scan.
Then you can install Windows Live OneCare.
Note, however, if you are using Windows XP 64 bit edition, OneCare cannot be installed to that OS.
-steve