Fast answer, yes if it used on the hardware it was purchased for.

 

OEM software is tied to the original hardware that it was purchased with, forever.

And you can do it all the times you want.

Yes, it work.

So Even with Formatting the Hard Drive it Works?

Does It Void Warranty?

Well, is not that void your waranty, but Dell will not support a second OS, so if you received the computer with Vista, and you install XP, they will not support your software. But you still have hardware support. For example, if your hard drive crash, they will replace it.

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But if you got XP and you were entitled to the Express upgrade, they support both, XP and Vista. And of course, if the upgrade is the one for the express upgrade, not retail.