Microsoft has warned of a "critical" security flaw in some versions of its popular Windows operating system.

The problem affects older versions of Windows such as 2000, Millennium, and 98, but not its newest version, Windows XP.

The flaw could allow malicious hackers to get into a computer, change webpages or reformat a hard drive.

The software giant has urged users to download a software fix from the company's security website.

Simpler language

Experts say the hole is particularly dangerous because existing worm viruses such as Code Red or Nimda could be altered to exploit it.

The flaw affects the Microsoft Data Access Components (MDAC), which provide database access for Windows.

This is the 65th security bulletin issued by Microsoft so far this year.

In a departure to previous practice, the warning was written in a simpler format.

Microsoft decided on the change as many users found its security bulletins were overly technical and confusing.

The company has also warned of a flaw in its Internet Explorer which could allow an attacker to see which sites you visit.

It said customers should update their version of the popular browser to close this security vulnerability.

New category

Since Microsoft boss Bill Gates launched his company's trustworthy computing initiative in January, the software giant has been far more open about warning people about security failings.

It has also added a new category of "important" to how it rates the severity of the flaws.

Microsoft now classifies vulnerabilities in its software as either "critical", "important", "moderate" or "low".

The flaw in Internet Explorer is the first to use the new category of "important".