You can reinstall by booting from the disk. You will, of course, lose any existing data, and you'll also need to reinstall any additional programs that you have.
After booting from the disk, you need to delete any existing partitions to ensure a clean install (I'm assuming that this is not an upgrade-only disk.
Frankly, I wouldn't have recommended upgrading in the first place. Windows 7 is very nice, and if you get it on a new machine, it works like a dream, but there is no compelling reason to upgrade an XP machine. The fact that you say that it ran slowly is somewhat worrying, because when properly installed on suitable hardware, Win 7 is (at least subjectively) faster than XP.