In windows 7
1. Close down the bitcoin client if it is running.
2. Backup your wallet.dat.
It should reside in C:\Users\Yourusername\AppData\Roaming\Bitcoin\wallet.dat
Copy it to a safe location.
3. Wipe out everything in C:\Users\Yourusername\AppData\Roaming\Bitcoin\
4. Copy back wallet.dat to C:\Users\Yourusername\AppData\Roaming\Bitcoin\wallet.dat
5. Attempt to start the bitcoin client again. If it starts again, then just patiently wait as the block chain is downloading again, this can take a very long time, so just let it run and be patient.
The bitcoin client should download all blocks and at the bottom it should show the same numbers as the official block count.
Number of blocks can be found here: http://www.bitcoinwatch.com/
, see under Blocks, count.
I would think that the errors you tell us about can come from corrupt data. If the procedure above does not work, then download the bitcoin client again from bitcoin.org and install it again.
Remember to backup your wallet.dat and copy it back though, so you don't lose any coins. When in doubt, always do extra backups.
Good luck, hope this helped!