Ok this one's pretty easy. step uno: go here: http://www.virtualbox.org/wiki/Linux_Downloads
select the appropriate file, download, and install either with gdebi or ubuntu software center (doesn't really matter which). or you can go into synaptic package manager and install the one in the repositories, they're slightly different but work the same as far as I'm concerned.
So once it's installed, you're gonna want to open it up, it's under "system tools". Once it's open, click "new" and choose your settings. For XP you choose "windows", "xp", and give it a name "Windows XP" is pretty creative. Anyhow, then after that you allocate how much memory to give it, click next, next, next, blah blah. Then it's created. Great.
Now you need an ISO to mount or your original Windows XP install CDs (haha yeah right). So you click on your windows xp virtual machine, click start or run or whatever (the thing with the green arrow), and a first-run wizard pops up. you can now tell it where to boot from (your ISO image), what size hard drive to give it, and a few other options. When all's said and done, it'll boot up and prompt you to install windows XP just as if it were installing directly to your computer. Once it's working, it operates exactly as if it were installed directly to your machine, so....yeah awesome right?