I just reinstalled Bitcoin-qt and exchanged the new wallet.dat file with my old one. The Address for receiving coins is shown correctly in Bitcoin-qt but the balance is 0.00 BTC. The Block chain is up-to-date and if I check the Address with Blockexplorer it says my Bitcoins are still on this Account. Why is the Balance 0 BTC in Bitcoin-qt?
Try this: start bitcoin-qt with the command line switch "-rescan". That should re-download all transactions, including those in and out of your wallet.
I can't guarantee this will work, but I had to do this when I upgraded to the newest version of Bitcoin-qt after a long time of not updating.