I recently found the private key from a wallet from 2010 when I generated a block of 50 bitcoins.  I imported that key into blockchain and the 50 coins showed up in my online wallet.  However, whenever I try to send coins, I get an error message: invalid signature.  I used the blockchain web interface from a PC for these transactions.  What's up with this?

I'm installing bitcoin-qt and currently waiting it for it to sync.  Should I try to import the private key there and see if that works?

I've read vague warnings about importing private keys, but nothing specific.  Can anyone provide details on the kind of dangers should I watch for?

Thanks

My Windows 7 machine recently crashed and after I restarted I now get this error when Bitcoin starts up:

EXCEPTION: 11DbException       
Db::open: Invalid argument       
C:\Program Files (x86)\Bitcoin\bitcoin.exe in CMyApp::OnUnhandledException()   

Of course my wallet file was not backed-up, but I do have the address.  Is there any way to recover the coins or am I SoL? 

Thanks