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Hi Doc,
The big difference in the 2 types is that the OEM version will most likely come without a fancy box and almost no documentation while with the Retail version you will get the fancy box, the extra documentation along with installation instructions, and other product information slips, etc. With the OEM version, it will most likely come in a paper & plastic sleeve along with the registration key on the sleeve and that will be about it.
So, if you have experience with installing a Windows OS before, I would say go with the OEM and save a little money. Otherwise, get the Retail version.
As to Pro or Ult versions, it all depends on what you want to do and whether or not you need all the "bells & whistles" or not. Read the chart that is on both the Pro & Ult pages and you will see there is very little difference. You will have to decide if you need the extras for about $40 more.
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