You need to protect yourself from Trojan attacks. Malware has become very sophisticated these days and any PC without an up-to-date robust anti-virus system installed is at very high risk. Computer infected with Trojans are especially vulnerable. you may be typing and sending your banking instructions to your online banking provider but that information may also be going, without your knowledge to some criminal somewhere.
The good news is that there are good free anti-virus solutions around if you do not want to fork out money for paid solutions. We do recommend however that you consider buying a solution. If you believe that your PC has been compromised, call your bank and have your online banking facility suspended until you clean up your PC.