Some of the keys on stickers do not work. In an attempt to thwart piracy, Microsoft invalidated many of the the keys on the COA stickers you find on branded computers. I have a couple copies of Vista OEM and a sticker to go along with it which is valid. However, if you are talking about something like Dell, HP, or something along those lines the key doesn't work.
The reason is that branded computers come with a disc that does not ask for a key and have automatic activation. The machines have some values embedded in the BIOS that is authenticated during install.
On an additional note, if you use a branded disc from HP or Dell, you can install to any machine with those discs, but you will find that when you go to system properties, it will also have the Dell or HP brand logo in there.