I am not so sure. A guy I work with tried the same thing (upgrade from an OEM), and apparently Microsoft asks for some kind of UAT or VAT number (?) when trying to upgrade. He was kind of ticked, but I later informed him that he can upgrade from Windows 2000 so he's just going to do that.
I'm fairly confident that your OEM copy will count, since most consumers who buy their PC's instead of building them will have OEM copies of XP preinstalled from the manufacture. MS would be silly and stupid to disqualify them from upgrading to vista. However, I could be wrong.... they might be counting on those people being willing to spend on money on a full retail copy and not an upgrade.
Well every machine that has been bought with a copy of XP will have an OEM copy on, I think it will cause far too much consumer confusion if they can't upgrade that copy because they don't (and really shouldn't have to) know what OEM means.
I still think the step up program could be a good idea but I haven't put it into contect and I haven't read any articles that seem to, but it despn't feel like something I can write myself as I could get things wrong, maybe if I run it past Nick White etc.
Basically - from what I feel - if you have WindowsXP Professional and you purchase the upgrade edition DVD to Windows Vista Ultimate, are you ripping yourself off?
Why? Well because you could have upgraded Windows XP Home Edition using the same disk and home is a fair amount cheaper than Professional.
Its a difficult thing to visualise as I keep seeing UK prices and US prices and even converting one to the other doesn't work properly as our prices have never been of the same value anyway.