I am a newbie and just installed Ubuntu 12.04 LTS. Instead of using the automatic dual boot option, I installed it manually on the partition made for it, but now only Ubuntu loads up with no option to boot Windows XP. Pressing F8 doesn't help either.
You had to use Ubuntu install procedure to install Ubuntu alongside the windows. Now, most probably your Ubuntu overwrited the boot and your Winodws are out of booting procedure. I would recommend you to fix the boot record from the Windows CD and install Ubuntu alongside Windows.
There are ways how to edit grub settings (do you have grub?/dualboot?), but if you are a newbie, could be a hard job for you.
...And you can try this: (https://help.ubuntu.com/community/Boot-Repair)