I have partitioned the HDD for XP and Vista. At the time of booting I have the option to select Xp or Vista. Now without formating the whole HDD i want to remove the Vista and in its place install UBuntu using a CD. Is it possible? How should I do it? I should have the option of selecting either XP or Ubuntu at the time of booting.
Have you looked at the Ubuntu installer? It has some pretty nice partitioning features. You can have it delete, create or resize partitions to fit whatever situation you need. When it finishes, it will install a new boot loader (grub2) and automatically add an option to boot XP. You could even resize the partitions instead of deleting Vista and triple boot if you want. As long as your hardware is supported, installing Ubuntu makes installing Windows look quite difficult, and you can try it from CD/DVD before you install.
I would suggest the latest version, instead of the long term supported version. There are Ubuntu releases every 6 months, but every couple of years, they have a version that they commit to support for 3 or 5 years, instead of 18 months.
If you decide you want to change your partitioning later, there is a CD called GParted you can download for free. You boot up to the CD, and it will move and resize partitions. It might even include a web browser and solitaire games, so you don't get bored.