There are a few options here, depending on how you initially set it up.
If you installed XP on a separate partition, you can simply delete that partition. After that you can delete the entry for XP in the boot tab of msconfig.
You can also go through that same process if you installed XP on a different disk.
If you installed XP and 7 on the same disk and in the same partition, deleting XP will be more difficult. The easiest thing to do would be to back up your data, format the drive and reinstall 7. If you don't want to go that route, you have to locate the XP directories and delete them.