In addition to 'XP is XP', XP runs only on Intel x86 CPUs (or x86-compatible AMD CPUs). Linux (and some other operating systems) run on many more CPUs, sometimes dozens.

What runs on one CPU must be recompiled to run on another CPU. The N800 uses the ARM CPU, which is not x86-compatible. Desktop Linux, on the other hand, is typically used on x86-compatible PCs.

Then there's also the GUI issues, which have been covered in the above posts already: XP has a single GUI/Window system, other operating systems often have a choice. That been said, almost every GUI/Windowing system on Linux are inter-operatable (that is, one made for one system will work fine on another, e.g. KDE<->Gnome). One of the few exceptions though is Hildon on the N800.. in addition to recompiling you'll also need to do a little bit of additional porting, at least to get a good result (e.g. works with stylus etc.)

Still, the hurdles are not that big, so new applications keep coming on the N800 (although the few dozen apps available for the N800 is quite a bit short of the 20863 applications currently directly available and installable (and free) for my desktop Debian Linux system..)