As others have said about OEM copies, if you buy it off the shelf it is a once use only. As in once you have installed it and activated the copy it cannot be reactivated again and if you change a major part of your system, vista will deactivate and you will be forced to either buy a new copy or licence! OEM pre installed version, this version can only be got if you have bought a computer from a company such as dell who pre install it at the factory prior to delivery. How the manufacturer does this is they put a licence key table known as a SLIC Table into its bios on the motherboard, so that every time you reinstall using the recovery cd/partition it installs the corresponding licence key files onto the system drive to match up with the bios. That then activates vista permanently without having to contact Microsoft. Obviously if you change the motherboard in this system you will no longer have the slic table and your copy of vista is useless unless you get another licence key! Upgrade, probably the best option as it is technically a retail copy that only lets you upgrade Via the licence key. You can upgrade to all the versions (Business not included) from XP Home edition. But if you are using Xp Professional you can only upgrade to Vista business! I personally have an upgrade version of Home Premium that I have activated more than twice with no problems. Finally Retail, this copy is the most expensive opition which can cost double what the upgrade costs just to let you install from scratch! If you have lots of money or you can get it for free this is the ultimate option (no pun intended). Hope this helps