1) Close bitcoin client (if it has been crashing you may wish to reboot or check task manager to ensure process isn't still running).
2) Open the Bitcoin data directory. https://en.bitcoin.it/wiki/Data_directory
3) Backup your wallet.dat if it has any value (of course you already have a backup right)
4) Delete* everything except the wallet.dat file
5) Start bitcoin client - it will download and re-index the blockchain (may take a couple hours)
* alternatively just move the files to another folder for temporary safe keeping