More than 15,500 computers in the UK are currently infected by a new powerful malware, with many more potentially at risk, the National Crime Agency said.
GOZeuS is thought to be behind fraudulent transfers of hundreds of millions of pounds globally.
"Our message is simple: update your operating system and make this a regular occurrence, update your security software and use it and, think twice before clicking on links or attachments in unsolicited emails," Andy Archibald, Deputy Director of the NCA's National Cyber Crime Unit, said.
Computer users are constantly being told to change passwords and update software due to the latest "Trojan" or "Bot" - but what are they?
The malware, called GOZeuS is hiding within email attachments, and once downloaded will enable hackers to mine infected computers.