Microsoft is warning its customers about a computer worm that exploits a flaw in its Internet Explorer browser.

A security bulletin on the company's Web site says Microsoft started investigating a "mass mailer worm," dubbed W32/Mimail@MM, late Friday morning.

The worm spreads through e-mail if recipients open an attached zip file — used to condense information so it can move faster over the Internet — then open an HTML file inside the zip file, the bulletin said.

The worm exploits a vulnerability the company addressed in a security bulletin issued April 23, the bulletin said.

On Saturday, Microsoft spokesman Sean Sundwall downplayed the worm's potential to cause major security problems.

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