Similar tale here. I built (assembled) a machine on an oem copy of Vista. I subsequently upgraded the sound card and on doing so, Vista announced that I was required to revalidate the install. I did so by telephone and was denied validation & put through to an operator who told me that as my copy was oem, I was not allowed to use it on two machines. I pointed out I wasn't & had simply upgraded a component, but was told that dfor the purposes of licensing, I had in effect changed to a new machine!
I will admit that I circumvented the requirement by my own means, but ms were wrong on the terms of their own licence for that copy & I did read the small print. They didn't make nice, so I navigated past them on my own terms.