When I was a college professor I used to be able to buy XP "upgrades" for very little (under $10). They were called upgrades, but they were the same as any OEM XP distribution. No problem installing them on a new drive in a new machine. I picked up a Vista a while ago -- also marked "upgrade" -- to try out on my MBP. Haven't done it yet.
I'd say chances that the "upgrades" are ordinary installation disks are quite good. How low is the price? If it's really low, it's worth spending the $ to find out.