Microsoft is warning computer users to protect their PCs after critical flaws were found in some versions of Windows, Internet Explorer and Outlook Express.
Two of the flaws are "critical"
It fears the security flaws could leave computers open to virus or worm attack.
Microsoft identified seven holes in its monthly bulletin which alerts computers users to problems. Fixes can be downloaded from its website.
Experts said home users were especially at risk because they do not tend to have the most updated security.
Microsoft announces any vulnerabilities in its software every month. The most important ones are those which are classed as "critical".
These are the ones which could be exploited by viruses. Often, when a critical flaw is announced, spates of viruses follow because home users and businesses leave the flaw unpatched.
"The gap between Microsoft announcing a critical hole and a worm exploiting that vulnerability is shrinking, with the Sasser worm it was just 18 days," said Graham Cluley, senior technology consultant for anti-virus firm Sophos.
"When Microsoft says there is a critical problem with its software, companies should sit up, listen and act."
Critical: Windows HTML help
Critical: Windows Task Scheduler
Important: Microsoft Windows Utility Manager
Important: POSIX Subsystem of Microsoft Windows
Important: Internet Information Services, Microsoft Windows
Important: User Interface, or Shell, or Microsoft Windows
Moderate: Outlook Express
He added: "Home users are particularly open to attack, because they don't often download the latest security patches from Microsoft, and may not be running a personal firewall.
"All computer users should ensure their systems are properly protected."
The critical flaws have been found in Windows Task Scheduler and Windows HTML help.
Both flaws could allow computers to be taken over and used as "zombies", executing remote commands.
Last month, net users were told to avoid using Internet Explorer because of a loophole which could allow malicious hackers to take control of a computer.
The US net watchdog, the Computer Emergency Response Center (Cert), and the internet security monitor, the Internet Storm Center, both issued warnings about the flaw in IE.
The warnings appear to have been heeded, with statistics from analysts WebSideStory suggesting that millions of people were switching to alternative browsers.