Vulnerability research firm eEye is warning of a critical flaw in Windows 2000 which it claims cannot be patched. 

Microsoft is currently investigating the problem and eEye is giving out few details in an effort to avoid helping hackers.

The flaw is thought to allow access to PCs by subverting the IP address. This would be ideal for a worm attack that requires no user interaction, such as Sasser. 

"While news of this latest Microsoft flaw is presently fairly opaque to the industry, we cannot expect that it is, or will remain, a secret. We can expect one or more worms to exploit this flaw as an attack vector very shortly," said Tom Newton, product development manager at security firm SmoothWall. 

Windows 2000 remains one of the most widely deployed operating systems in corporate IT environments. Microsoft no longer supports the software and issued its last patch in June.