My bitcoins are completely secure. They are encrypted with an unbreakable password. They are backed up in many places globally. The only way to take my bitcoins would be under duress and even then you would only get my small stash. Nothing comes even close to the security of bitcoins.
However, an attacker only has to install a small program on your computer that waits for you to unencrypt your wallet (since you eventually have to use it for it to mean anything) to steal your private key then.