Sure. The basic procedure is to backup all your personal data onto an external disc or drive, then boot to the XP install disc. When you get to the point where it asks you where to install XP, you can delete the existing partition or partitions and then create a new partition and install XP on it. Then install your drivers, then security, do all the windows updates, reinstall all your software, then restore your personal data.
But instead of downgrading to XP I recommend that you look at getting a copy of windows 7. If your computer runs Vista, then it will run Windows 7 well. It is to Vista what XP was to ME. I installed Windows 7 onto my laptop and it runs much cooler and faster than it did when Vista was installed.