I'd do what was suggested herehttp://bitcoinstats.com/irc/bitcoin-dev/logs/2011/07/08/2
1> backup your wallet.dat file. (default location in windows is %appdata%\bitcoin\wallet.dat)
2> delete all the files in the directory where the wallet.dat lived (you do have a backup copy of wallet.dat)
3> copy the wallet.dat back
4> upgrade to the latest bitcoin client (http://www.bitcoin.org/
5> run it to redownload the entire block chain
(don't panic, it will initially show your balance as 0.00 BTC until the full block chain is downloaded)
Or this may help or be a total waste of time, but even if it fails it may give you a more detailed error message.
1> backup your wallet.dat file (it is always a good idea).
2> start a DOS command shell
3> change directory to the folder where bitcoin.exe is installed (probably "cd C:\Program Files (x86)\Bitcoin")
4> and run bitcoin by hand
"bitcoin -rescan" this should rescan the block chain for missing/corrupt wallet transactions.