You can install Windows Vista 32 Bit as an upgrade however I do not recommend to do so as the performance is usually quite a bit worse if "upgraded" via "upgrade".
I bought a Dell Vostro 200  in 2008, using XP Pro as the operating system.   Apparently, Vista Business 32Bit SP1 is also already installed on this PC -- I have a Reinstallation DVD for that. How can I switch now from XP to Vista?    I have no clue as to where it is or how to access Vista.   Problems to watch for? Many kind thanks to the Dell community for your help. K 
How to switch to VISTA already installed after using XP Pro for 5 years