Place bootstrap.dat in the bitcoin data directory and then start Bitcoin-Qt. It will read the file to create the blockchain. The file will be renamed to bootstrap.dat.old when this is completed to prevent reprocessing when Bitcoin-Qt is restarted.
You can locate the data directory by reading this.
From : http://bitcoin.stackexchange.com/questions/19097/connecting-bootstrap-dat-to-bitcoin-qt
You can use the bootstrap.dat torrent with no ill effects regardless of your current block. When you restart and Bitcoin sees the bootstrap.dat that you have dropped in it's data directory, any block hashes you already have will be skipped quickly, and the import will pick up at the blocks you don't have.
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