Hi all.
I have both Ubuntu and Windows XP installed on my machine; although I correctly have to choose between XP and Ubuntu after boot, I'd like to have Windows XP as the default option (the higlighted one) instead of Ubuntu, as it is a shared machine and I'm the only Ubuntu user.
Thanks in advance.

I don't know which bootloader Ubuntu uses, but you need to edit the relevant config file: menu.lst for GRUB (found in /boot/grub usually) or lilo.conf (on Slackware at least, this is in /etc) for LILO.

For GRUB:

There's a line in the file that says "default x", where x is the number of the default entry. Counting begins from 0, so the first entry in the file would be 0, the second would be 1 and so on. Simply change the number to whatever corresponds to your Windows entry and save the file.

For LILO:

The relevant line in lilo.conf is "default=label", where "label" corresponds to whatever string the "label=" line of your Windows entry is set to. After making the change, save the file and run "/sbin/lilo".

HTH.

on the grub screen there are some key options you can press to change defaults n such