How to Delete one of my VistaOS?

Hello,
Hi.. I would like to know how to Delete one of my VISTA OS on my TOSHIBA laptop whereas i have two VISTA OS in it.. one of it is old and the other is new. and i just want to use the Old Vista since i have all my files in it and delete the other new Vista because i think it is leading to sluggishness while using the PC.. While starting the comp. i get the option to choose which one to Load. So i want to delete my New VISTA OS and keep the old one still.. So how do it do it??

plz do reply to my post and hope ur able to solve my problem.

Thanks in Advance.
Cheers!