A new vulnerability has been discovered in Internet Explorer versions 6 to 11 which has already been subject to some attacks.
Microsoft itself issued the security alert, saying that it's looking into how bad the hole is in order to decide whether a dedicated security update to the browser is needed - if not, the hole will be patched in the next monthly security update.
Update: The US and UK governments' Computer Emergency Readiness Teams (CERT) have advised people to stop using IE until Microsoft fixes the breach, particularly if you are on Windows XP.
Remote code execution
The vulnerability may allow an attacker to get access to your computer, files and user rights - and "limited, targeted" attacks to do so have already taken place.
It sounds as though the attackers would need to get you to click on their "specially crafted" websites in IE before they could gain access to your computer - so steer clear of any dodgy-looking links.
If you're using Internet Explorer 6 to 11, you may also want to switch to an alternative browser until Microsoft sorts the issue out - particularly if you're on Windows XP as there will be no security updates for the aging OS.