I just answered your PM but I'll post here as well, in case anyone else wonders about the wonderful world of Linux! When you install Ubuntu it will install a boot loader (Grub or LILO are the two most common; Ubuntu uses Grub), and this is what allows you to select which OS to use after you start your computer. If you installed Ubuntu WITHIN Windows XP, though, it won't do that - you run Ubuntu through XP, while XP is running, just like opening any other application in Windows. So if you want the ability to select which OS to boot to instead of running Ubuntu through XP, just boot from the Ubuntu CD and during the install, you will partition your hard drive for Ubuntu to reside on and it should automatically setup the XP install to boot from in Grub as well. In the past, you had to manually setup the bootloader for XP, but Ubuntu makes it very easy and does it all automatically. Hope this helps! Ubuntu rocks! :)

There is a third option for installing ubuntu in Windows Xp.. Its called wubi installer. Basically its like installing a program on windows xp, for example office 2003. Where if you don't like the program you can uninstall it simply through add or remove. When you install ubuntu through wubi, when ever you start up your computer you get two choices, windows xp or ubuntu, and I believe windows is default choice. So its really easy to install and play around with, plus windows xp is only a restart away.