Sure it was sent by your client, or did you store your wallet.dat somewhere else as well?  Look up the transaction ID in blockexplorer, and search for the first 20 bytes of it in debug.log.  (It is in ~/.bitcoin/ under Linux.  Have no idea where Wintendo stores it.)  If you see an "askfor tx transactionid" somewhere, the transaction was initiated by someone with a copy of your wallet.dat.  It looks like your own, because you have the same wallet.