whatever & wherever it does not matter,
but the procedure should be as follow:
1 – Turn on your computer and when wondows inteface show put the windows Vista in CD-ROM.
2- It will show you windows vista interface with alot of options.
3- choose the 1st option that said install windows vista by clicking on it.
4- this is the important part , it will show you a selection bar and this bar have two options (new installation) & (upgrade)you will choose new installation and click next.
5- follow the windows installation instrauctions exactly like install windows normally.
6- when you done with the installation issue, everytime when you will turn on your PC it will show you two selection , one for windows XP and the second windows vista.