The trouble here is that MS have 'turned-off' the activation of keys that are supplied via SPL installs.
Basically, in laymans terms what this means is that any key on a branded machine will not activate online- You will have to use the restore disks or ring MS.
Where the vendor supplied a full hologram CD, then it should activate online as long as its the same machine etc.
The quickest way to tell I believe is to look at the COA sticker- Blue OEM & it won't activate online, otherwise it should.
(Also, use something like Magical Jellybean to find the XP key from the install- It will be totally different to that printed on the bottom)
-Leezer-