Software giant Microsoft has warned of seven security flaws in its programs, describing five of them as "critical".
Microsoft plans monthly security alerts
The alert applies to a wide range of Microsoft software and the critical flaws could let hackers break into computers.
The announcement is part of an initiative to tell computer users of patches on a regular basis
People are being urged to go to the Windows update site and download the latest fixes.
The software affected by the latest flaws includes Microsoft's Exchange e-mail server, various versions of its operating system, the Windows Messenger service and multimedia software for web browsers.
Some of the flaws could allow hackers to access computers through a malicious program similar to the damaging Blaster worm.
But Microsoft said it did not believe hackers were yet exploiting any of the vulnerabilities.
The alert is the first of the Microsoft's monthly security updates. Last week the company said it would limit the announcements to once a month, except in emergencies.
The aim is to make it easier for people to update their software in one go, rather than applying individual patches on an ad hoc basis.
It is part of Microsoft's Trustworthy Computing initiative, which is designed to make its programs more secure and reliable.
Critics say the company should design better software from the start, rather than rely on updates to fix security problems.