A new worm is spreading over the internet that takes advantage of the widely reported Windows file-sharing flaw.
The flaw was patched by Microsoft at the beginning of last month, but the SANS Internet Storm Centre and various security companies are warning that unpatched systems are being successfully targeted by the worm.
Microsoft issued security bulletin MS06-040 to tackle the problem as part of its monthly scheduled security update.
It has also prioritised distribution of this patch as it says the vulnerability is more serious than other flaws it patched at the same time.
The US Department of Homeland Security also issued an alert about the serious nature of the flaw soon after the patch became available, but sluggish users are still leaving their systems open to attack.
It is estimated that there are now six different malicious programs being circulated on the web that take advantage of the Windows vulnerability.