It seems blockchain is corrupted...
1)
backup your wallet to a safe location(the most important part
)
the wallet (wallet.dat) can usually be found:
C:\Documents and Settings\YourUserName\Application data\Namecoin(Windows XP)
C:\Users\YourUserName\Appdata\Roaming\Namecoin (Windows Vista and 7)
2) delete all files except "wallet.dat" from
C:\Documents and Settings\YourUserName\Application data\Namecoin(Windows XP)
C:\Users\YourUserName\Appdata\Roaming\Namecoin (Windows Vista and 7)
3) restart namecoin client and wait until synchronization will be completed.