Microsoft is due to issue a patch to fix a security flaw believed to have affected as many as 10,000 Web sites.
The emergency patch should be available from 1800 GMT on 17 December, Microsoft has said.
The flaw in Microsoft's Internet Explorer browser could allow criminals to take control of people's computers and steal passwords.
Internet Explorer is used by the vast majority of computer users and the flaw could affect all versions of it.
So far the vulnerability has affected only machines running Internet Explorer 7.
According to Rick Ferguson, a senior security adviser at security firm Trend Micro, the flaw has so far been used to steal gaming passwords but more sensitive data could be at risk until the security update is installed.
"It is inevitable that it will be adapted by criminals. It's just a question of modifying the payload the trojan installs," he said.
It is relatively unusual for Microsoft to issue what it calls an "out-of-band" security bulletin and experts are reading the decision to rush out a patch as evidence of the potential danger of the flaw.
Some experts have suggested that users switch browsers until the flaw is fixed.
Firefox, Opera, Chrome and Apple's Safari system are not vulnerable to this current flaw. But Graham Cluley, senior consultant with security firm Sophos, said no browser is exempt from problems.
"Firefox has issued patches and Apple has too. Whichever browser you are using you have to keep it up to date. People have to be prepared and willing to install security updates. That nagging screen asking if you want to update should not be ignored," he said. More PlayStation 3 News...