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1) What file do I need to edit to change the default OS from Linux to XP?



/boot/grub/menu.lst
(but there's probably a GUI somewhere to do it with less risk).
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2) And I assume I need to edit something in the same file to add an option that allows me to go to a prompt rather than starting xwindows?


That kind of thing was normally done using "run levels", but Ubuntu doesn't use runlevels that way (redhat does, though).

Do you really want to not START the GUI, as opposed to not LOGIN-TO the GUI?
(Eg. hit CTRL-ALT-F2 after the GUI appears).
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3) What partitions of what size should I have created when installing it?


The installer default is what you should have used.

One big root partition, one smaller swap partition, period.


Cheers