If you download it from a third party, you should verify that it’s the true file. The MD5 for the file is 59a98f181fe383907e520a391d75b5a7 and the filesize is 278,927,592 bytes.
SP2 is making its way into Windows Update and the Automatic Updates feature. If you want them to ignore SP2 temporarily (to verify compatibility, etc.), there is more info here.
The executable creates the registry key and sets the associated value on the machine upon which it is run, to block or unblock (depending on the command-line option used) the delivery of Windows XP SP2 to that system, through Automatic Updates or Windows Update. The key used is HKLM\Software\Policies\Microsoft\Windows\WindowsUpdate.
Whe n the /B command line option is used, the key value name ‘DoNotAllowXPSP2’ is created and its value sets to ‘1’. This value blocks delivery of Windows XP SP2 to the machine through AU or WU.
When the /U command line option is used, the previously created registry value that temporarily blocked the delivery Windows XP SP2 to the system via Automatic Updates (AU) or Windows Update (WU) is removed. If the value does not exist on the system from which it is run, no action is taken.
It’s just a registry key that causes WU and AU to temporarily ignore SP2. This is an easy way to put off installing SP2 without having to change a bunch of stuff on your system. However, if you have no major compatibility issues, you really should upgrade. There are many security enhancements that will help stop problems in the future.