There's word that Microsoft is scrambling to fix a previously unknown flaw in its Windows operating system.

The Washington Post reported the problem leaves computer users vulnerable to spyware, viruses and other programs that could take over their machines.

The hole "is being actively exploited" the company said in a warning posted online.

The software giant said it is investigating the problem and will put out a software patch to fix it -- but does not say when.

The Post quoted Microsoft Security Response Center Operations Manager Mike Reavey as calling the flaw "a very serious issue."

Microsoft said the problem is in the portion of software that renders graphics. It said the code is being used by people who place malicious Windows Metafile images on Web sites.

The warning points out that users must be lured to a site with the file, possibly by clicking a link in an e-mail, but can't be forced there.