Windows users are being urged to install the June package of security patches from Microsoft.
The security update contains six patches which close 15 vulnerabilities found in a variety of Windows programs.
The patches help to shut loopholes in Internet Explorer, Outlook Express, Windows Mail and Windows Vista.
More than half the loopholes are rated as "critical" which often means they can be exploited without a PC owner's knowledge or consent.
Malicious code to exploit one of the vulnerabilities in the package is already known to be circulating online.
Using this code hi-tech criminals could craft e-mail messages or booby-trapped webpages that could take over PCs.
Some of the vulnerabilities fixed by the patch rated as "critical" are for various versions of Internet Explorer including the one that runs on the Vista operating system.
Vista, the latest edition of Microsoft's operating system, gets a patch for itself which aims to stop malicious hackers exploiting a loophole to steal administrative passwords.
Included in the bundle is a patch that tackles a serious problem in the way that Windows handles part of a program called the Windows Secure Channel.
This vulnerability could be targeted by malicious hackers as it is used by many websites to handle security certificates and keep user data confidential.
Windows users can get hold of the patches by turning on the automatic update feature in the operating system or by visiting the Microsoft Update site and downloading them.
The second Tuesday of every month is the date on which Microsoft typically issues security patches for its software programs.