How do you DUAL BOOT k800i?

hello there.
i took a look at some Youtube vids of modded k800i's, and i don't know how, but they seem to beable to switch complete transformation of there phone in an instant.
there was this guy that had a walkman theme... he guided through it then turned his phone off, turned it back on again, selected something from a startup menu (it was in a different, language, so i don't know what it said.) and it loaded a windows Xp splash, then his whole
phone loaded up into a Xp theme, with menu icons and everything. thats SOO cool.
how do you do it?

CHEERS

could you link us too?

i'll like to see that video too, pls link us.