BitPay explains that the Trojan.Coinbitclip virus attacks a computer's clipboard. When the bitcoin user copies and pastes the bitcoin address of his intended recipient, the valid bitcoin address is changed to a bitcoin address controlled by the attacker who introduced the virus. The attacker's bitcoin address (which often looks very similar to the original address) receives the funds.
“This virus has not compromised any wallets or payment systems. Instead, it's introduced into Windows computers through malicious software or email attachments”, BitPay added.
http://www.econotimes.com/BitPay-Cautions-Users-Against-Trojan-Virus-Affecting-Bitcoin-Purchases-196234