Just also make it clear about the difference between Retail and OEM. OEM has a licence which is tied to one machine and if any major parts are replaced ( a motherboard is considered major) then the licence is void and a new one is needed. Retail however can be tied to one machine after another ( only one at a time though!) it's more expensive to buy initially but is cheaper in the long run if you expect to be upgrading often.
An upgrade version will only work if another copy of windows is found on the disk prior to install eg If you bought a new HDD and tried to install Vista (or any other windows os) upgrade version it won't install because it first searches for the previous copy. It can fooled however.. If it doesn't find a previous os it will then ask you to place your original os in the drive. If you have an old copy of 98 (even a upgrade version lol) it will be excepted and the install will go ahead. ( I used to do this with a upgrade version of XP I once had)

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