Open your bitcoin-qt client and leave it running for a day or two, it should download the blockchain automatically and be synchronized within a few hours to a few days.
If that doesn't work, try using the "rescan" launcher from my "BTC/LTC Newbie Tools for Windows" package:
https://bitcointalk.org/index.php?topic=113707.0The rescan option will usually fix frozen blockchain downloads and/or missing blockchain info/transactions.