It depends what you mean
1) As an example, Is your machine running windows and storing bitcoins in a wallet, open to being attacked? yes, definately, the same as hackers stealing money from your internet banking. A virus or trojan once installed can easily steal your coins. But this is not a vulnerability in Bitcoin.
2) The Bitcoin protocol seems resistant to attacks and hasn't been compromised yet. Individual node machines that are compromised would be ignored by the rest of the network. So I would say Bitcoin itself is not vulnerable to viruses.
However it may be vulnerable to a DDoS attack, I guess time will tell.