I am not clear as to the "automatic" compact process you are referring to. XP SP1 had an automatic feature where Outlook Express would compact the folders in the background whether you liked it or not. Upgrade to SP2 and this feature will be removed.
On the other hand, XP SP2 will put a message on the screen and will prompt you to compact your folders and you can choose to do so or not. If this is what you mean by "automatic", sorry, there is no way to stop this. Every hundred times that you open and close Outlook Express, the message to compact will pop up. It has an unstoppable internal counter.
Actually, compacting the folders in Outlook Express is pretty vital since as a fairly primitive database, Outlook Express can really mess up. That's why Microsoft puts this irritant in there.
But, all is not lost. You can download this little program. It is free:
http://www.oehelp.com/OETool/ One of its features is that you can manually compact your folders and it will reset the counter to 0. At least if you compact when you want regularly, you will have reset the counter, and theoretically never have to be surprised by the rude little message.
By the way, never try to stop the compacting process once it has started. It can ruin your pst file.