If you have apps that require XP, you can try to use Virtual XP mode. You need to download it from MS site.
If you really want to install XP, you need first create recovery dvds (in case you want windows 7 back) and reformat the drive and do a clean install. Need install CD. Remember that reformating drive = losing all personal data.
I've never done this but you should be able to simply re-install XP from disk normally. You'll be asked if you want to repartition & re-format. I would do so and that will be the end of W7. The rest should be a normal XP install. Alternatively, you can always install XP in a second HDD and use the W7 drive to migrate data when finished. BTW, XP is schedueled to be supported through 2014, I believe.