if you have a laptop with a powerPC processor in it (G4), you can use virtual PC to emulate certain windows programs. But it'll be slow, it doesn't replace an actual windows laptop.
with the introduction of intel processors steadily covering apple, people are working on getting an emulation layer to be able to get more windows programs working under mac.
currently there is no way to get windows XP to run on an apple laptop. it's likely to be possible with lots of messing about with the latest intel laptops apple have launched, but no-one has managed it yet. the new laptops use a different system than the normal bios used by XP. vista can use this too, so it'll either be vista running on the apple, or using a tool from vista to get XP running.
for the latter option though; why would you even want windows running on an apple laptop? it's like getting a ferrari and putting an old skoda engine inside.