I know this story has been around since last Friday, however it wasn't until now that I could find official reference on Microsoft's website. I guess MS did it big this time, fortunately I haven't suffered from such slowdowns in all this time I've been running my patched version of XP Pro.
After you install Windows XP Service Pack 1 (SP1), your programs may take as much as ten times as long to start.
This problem may occur if the programs you run frequently allocate and deallocate large blocks of memory. Changes in Windows XP SP1 in the memory management system have caused this operation to take significantly longer than with pre-SP1 Windows XP.
Microsoft is offering a fix to those severely affected, the patch is not available for download but instead you need to call MS' Product Support (at no cost) in order to obtain the fix, otherwise you will have to wait for SP2 to arrive.
Check my reply to this thread, the file can now be found online / Per