All you have to do is to use custom option and check whether your present window uses 32- 64 bit.
To check your whether your computer is 32 bit or 64 bit follow the steps-
* Right click on the desktop and then click on the properties.
* Then check in the dialog box whether you have 32 bit or 64 bit configuration.
* If having 64 bit then enter the same configuration in the vista control panel while upgrading it to vista.
Moreover,also require additional hard disk and RAM to increase the capability of your Window XP configuration and enhance its performance so that it is fully compatable with the hardware configuration as well.