If you think it is possible that your wallet is compromised it is best to create a new wallet and transfer the bitcoins there. This means if it is possible that someone obtained your wallet file or encrypted wallet password. The old wallet should be saved in case bitcoins somehow get sent to an old address.
With Bitcoin-QT the wallet file is called “wallet.dat.” Shut down Bitcoin-QT and rename the wallet.dat file. In Windows it is usually at C:\Users\username\AppData\Roaming\Bitcoin\wallet.dat | Linux: ~/.bitcoin/ | Mac: ~/Library/Application Support/Bitcoin/
Just restart Bitcoin-QT and a new wallet will automatically be generated.