This is not XP replacing your windows mobile os, but a virtual XP installed on your SD card. USB Mass Storage device emulator such as card export, or wm5torage is required.
Install Mojopac to your card, run it, install all your favorite progs (your usb mass storage drive becomes your c: drive while inside your Mojopac), and use your portable XP on your friends, or families computer. When you unmount your mojopac drive (eject), no traces of your use are left behind.
Some real geeky stuff, i suggest you geeks check it out. Minimizes the lugging of your laptop. Best of all, it's free, or at least for now it is.
http://www.mojopac.com/portal/content/hellomojo.jsp