You should be able to download the standalone file from any computer: www.microsoft.com/.../details.aspx  
Can't download Microsoft XP SP3 for another computer.  How do I do this on a system that has it already?  I just want to put it in a folder and copy it onto my other machine.  Everytime I try to download it I get an error message when saving to disk.  I can't connect the machine I want the SP3 on to the internet without SP3 according to Comcast. The machine I'm trying to update had a hard drive crash, and I installed a new HDD, reinstalled the Dell XP SP2, and reinstalled the Dell drivers.  The cable high speed internet no longer allows manual config of the networking due to dynamic IP.  I can't connect that computer to the internet to download the SP3, so I need to download it on another computer and copy it over.  However, I can't get the download to save to disk, probably because the machine I'm attempting it on already has SP3 on it.
Problem downloading XP SP3 for another system.