A fresh unpatched security flaw has surfaced in Windows 2000, impacting the many end-users and companies still dependent on the operating system.

The flaw affects a core component of Windows 2000 that can't be disabled, meaning a workaround is unlikely, according to security firm eEye, which discovered the bug.

The company said it won't release details of how the flaw works, until Microsoft has issued a patch. However, it said the vulnerability isn't limited to Windows 2000 alone - Internet Explorer, Windows Server 2003, Windows XP and XP SP1 are all vulnerable, its advisory states.

EEye gave the bug a "high risk" rating, and said it could be easily exploited, by a worm for example, to take over systems remotely.

Microsoft said it is investigating the bug and may issue a fix. However, Microsoft is winding down support for Windows 2000, despite the fact that it is still widely used in enterprises. Mainstream support ended in July of this year, following a final update on 28 June.

Support for Windows 2000 may be drying up, but the security flaws aren't. This year so far, 22 security vulnerabilities have surfaced in the operating system, many of them allowing remote system takeover, according to independent security firm Secunia. Last month, Microsoft patched such a bug in Windows' Microsoft Windows Color Management Module, for example.

More worryingly, a serious flaw that surfaced in April, affecting the Jet Database Engine, still hasn't been patched, according to Secunia - despite the fact that exploit code has been posted to a public mailing list.

The Jet Database Engine flaw affects software such as Windows XP SP1/SP2, but since XP is based on Windows 2000 many such bugs also affect the older operating system - but from now on, won't be getting the same access to patches.