How can I remove Windows XP from my computer?

It depends on haw you have installed ubuntu. If you installed it inside windows, as another application, you can't uninstall windows and continue to use that ubuntu installation. In that case you have to install ubuntu in another hard disk partition.
If you installed it in a special partition:
1)Once you have a backup of every file you want to keep, you couldboot with ubuntu, open gparted, and erase the windows' partition. Then you can add that harddisk space to the ubuntu partition, or create a new one for data storage, or another operative system. There must be also possible to uninstall windows from the inside -- i mean, it must have an "uninstall" option, though I've never seen it (nor looked for it nor heard of it). But if you don't need anythig else from that partition, you might well format it (and install another OS) or erase it (and add it to the ubuntu one).

2) While "1" answers your question (if it doesn't, or at least not clearly enough, let me know), "2" consists in some advice. If I where you, I wouldn't uninstall windows. As a matter of fact, I didn't. I installed ubuntu in a second hard disk, and kept windows, although i hardly ever use it. Even if you always use ubuntu (as i do myself), a second OS can be useful. You might need to use some windows-only-program one day, or maybe someone else needs to do it or you want to let them use your computer, and they might want to use windows if they're not comfortable with a strange OS. Or if for some reason your ubuntu installation is destroyed, and you need to >>use a computer<< immediately, with no time to install ubuntu again or solve its problem.
With gparted (from linux) you can resize partitions, leaving less space for windows and passing more to ubuntu.
If you are short of hard disk space and can't afford having two operative systems, or just want to get rid of windows anyway... well, you're the boss. But remember: a second OS already installed is a good insurance; windows, although very inferior to ubuntu, is somewhat useful; and it can't damage you, in the end.

Hope you find this useful. Sorry if i was too long.

+1 to deleting the partitions but all warnings are perfectly valid. You could even use the space to create a seperate partition for /home or simply use it as a data dumping ground. The partition will need to be mounted using /etc/fstab but you will then have a folder representing your newly harvested space that you can use to store your goat porn / knitting patterns / pictures of tennis balls etc.