XP in my computer is deactivated

I just bought a used laptop and the xp just deactivated after 30 days...i got the COA on computer by using snapfiles.com and when I attempted to activate, I was told the key was unauthorized.
Do I have to buy a new xp cd w/coa? ...Any help would be appreciated as I am not too familiar with this. By the way, I cant get in touch with seller.
Thanks!

Contact Microsoft directly by telephone. Let the know the situation and they should sort you out a new key. If not, legally you might need to repurchase a key.