Since none of the PCs for which I am responsible has "Microsoft Dynamics Retail Management System", a Point Of Sale application, installed, I thought I might as well update them.
So far I have installed SP3 on 19 work and home PCs and laptops, which has taken between 11 minutes (for the newest Dell Optiplex 330) and 62 minutes (for an ancient just-about Pentium IV PC with 256 MB of RAM) from start-of-install to receiving the machine back usably after the reboot. No problems were experienced, except that the Windows Scripting Host command prompt window which appears during the post-reboot startup occasionally displayed well over a dozen identical error messages in rapid succession, rather than the single word "Done" which appeared on the other PCs, before terminating, apparently normally. (In fact you get two command prompt windows, but I've never seen anything displayed in the second one.)
Thankfully, I encountered no instance of the snappily-titled Windows XP SP3 BSoD Endless Reboot Loop with Stop c0000139 on GDI32.dll Error.
This information is just to give some idea of how long the SP3 installation process might take on your PC when SP3 again becomes available, once Microsoft has put a check in that your PC isn't running the above POS application!
Thanks to Alan for the original link to obtain SP3 (it was a 316.4 MB download, by the way)...