It sounds like your data directory may have a corrupted blockchain or other files.
The data directory is %APPDATA%\bitcoin, which typically resolves to C:\Users\username\AppData\Roaming\Bitcoin. Before we get started, copy your wallet.dat file in that directory to a USB stick and store it away, and copy the entire data directory to a new location, name the folder bitcoin-backup or such, so you can always put things back as they were before you attempted repair.
The easiest way to rectify this situation is to remove all files from the data directory except the wallet.dat, download a nightly version of the the blockchain from http://bitcoin.bluematt.me/bitcoin-nightly/blockchain-nightly/
, unzip these files into the bitcoin data directory, and launch bitcoin again and let it update to the latest block.