Dual Boot - already loaded - need to remove Vista 64 to 32

I have XP home installed along with Vista 64 (big mistake). I would like to take off Vista 64 and replace it with Vista 32. My hard drive is already partitioned and I already get a choice of which OS I want to use. I just need to change from V64 to V32. I want to keep XP on just in case though.

Ques 1 - Do I need to format the drive and simply load 32 on?
Ques 2 - Can I just do a add/remove programs and then install 32?

I jumped the gun when loading Vista 64 as it clashes with everything including the weather. LOL. I am hoping that installing 32 will be a better choice.

Any tips or suggestions on the smoothest way - Rookie here! Thank you in advance as all advice and suggestions are appreciated!