A lot of things cause slow XP loading times.
Memory is the main one... remember that XP uses alot, and for optimal operating speed of it, you should have 512Mb (my friend has 256Mb, and it takes about 10mins to load, as well as it being slow elsewhere). XP eats memory for its brekafast.
Having loads of fonts installed can make it slow on boot-up too.
Also I have heard people say that keeping the HDD defragged helps it go faster, but then an equal number of people say defragging makes no difference to the computers performance.