Make sure the Bitcoin client is not running and then go to the directory "C:/Users/"your username"/Appdata/Roaming/Bitcoin" and move the files blk001, blk002, and blkindex to My Documents. You will have to show hidden files and folders. (if you don't know how to do this just search show hidden files and folders in the start menu) Then start the Bitcoin client and wait for it to re-download the blockchain.
The block chain is over four GB so leave the client running overnight. This is a semi-common problem, and the only way I've ever heard to fix it was to either let the Bitcoin client re-download the blockchain, or to download and replace the blockchain manually(This is more difficult yet faster and more reliable fix.)