you can use Ubuntu in Daul Boot with Vista (ie when you turn on Pc it will aks what you want to use), wiht version 8.04 and newer you can install it onto Vista using Wubi, this menas Ubuntu runs liek a progmra, you jsut open and close it while using Vista.
If your unsure of what it is then you cna use it as a "LiveCD". This means you cna test it out off a CD, so no need to even install it, and if you dont liek it you jsut dont install it.
If you want ot run it dual boot then in Vista go to-
Start - Right Click 'Computer' - Manage - Disk Managment - right click on the bar at the bottom(should be C: drive) and choose 'Shrink Volume' - choose a decnet amount, about 30GB shoudl do, you need 10GB minumum, but 40Gb lets you have space to install programs.(you cna use hte space on the Vista partion for documents, mucis videos etc)
A new version of Ubuntu is out in october, so i'd wait till that is out.
(they come out every 6 months, last was April 2008(8.04) next is October 08(8.10) they sue the date for the version number.