You can wipe out Vista and install XP again, but since Microsoft does not support 'downgrading' Windows performing such an operation will erase all of your personal files, installed programs, etc. forcing you to start fresh. Be sure to backup anything you cannot afford to lose before proceeding and give yourself time to restore everything.
I cannot abide Vista with all its incompatibilities. Can I use an OEM version of XP and install over the Vista? Usually, when you install an OS, it asks you whether you really want to do it as it will format the HD. Can I go backward?