Speaking from experence, one of the gotya's of XP is that through time, the boot up time becomes glacial and I do mean slow. Vista is much the same. I wonder how WIN 7 fares in this respect ? - From what I have read, its fundamentally an improved Vista.
CAT III
That's not XP per se, that's the software you install on top of it which 'requires' a memory-resident portion because people seem to have lost the ability to code properly any more.
A vanilla XP install, properly defragged regularly, and containing no new software, won't change in bootup speed at all over time.