XP is a multi-user OS. Its built on the Win2000 base rather than 9.x base. I can log into my xp box from my mac, while my flat-mate continues to use the xp box logged in under his account without interruption. You can have muliple users logged on to an XP box at the same time and using the comp at the same time. Just coz they cant do it at the same terminal at the same time doesnt make it single user.
XP's interface is faster than OS X even on similarly specked hardware. (i have a 1Ghz PC laptop and a 900Mhz iBook, they both have a similar amount of ram) The eXPerience, however, degrades through normal usage unless the machine is maticulously cared for, (i.e no installing and unistalling of aps etc.) OS X on the other hand doesnt degrade in speed.
Also interface speed doesnt mean the XP box (on similarly specked hardware) will do anything faster. (Copying/moving files, running scripts etc) OS X also remains more responsive during heavy tasks than does XP.
XP's interface is also less eye-candy heavy than OS X, much fewer and lower quality animations and effects. None of those shadows and transparent bits and bobs. So its hadly suprising that it feels snappier(
TM). I like the eye candy tho

If you skin xp (using 3rd party apps) to have all the animations etc it is much much slower.
XP is still bad about releasing mem. Its a lot better than 9.x but still behind OS X. after a few days use, (launching and using word, excel, photoshop etc, ) XP benefits from a reboot, whereas OS X doesnt.
Just my own experiences. I used an XP laptop as my main machine for about a year.