OEM Vista worth it?

The only Big disadvantage for OEM is that it will be tied to the hardware it was installed on first, somewhat limiting your upgrade path. You can still upgrade components, but if you change big stuff like a mainboard and CPU, MS might refuse to activate it again. Installing will be exactly the same as a retail version, and isn't very hard (just boot from the disk, select the partition to install on and enter your license key, that's all). I got the OEM version of Ultimate since it was less then half the price of a retail, and I figured; If I am gonna upgrade my PC to such an extend that I'm replacing a mainboard, I am gonna need an extra license anyway as I'm not throwing away the old stuff. A retail version can migrate to a new PC, but you cannot have it installed on 2 pc's at once, so in my case that was no good anyway.

BTW, there is a manual with the Vista Ultimate OEM, with all the basic functions explained, so I guess the big difference there is the MS 90 day support (not worth double the price IMO).

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That is exactly the way I thought about it, especially since MS announced that the next Windows version could come as early as mid 2009, 2.5 years after Vista, rather than the 5.5 years it took between XP and Vista...