A major update to the Windows XP operating system will be available to download from August, two months later than originally expected.
Most of the world's PCs use the Windows operating system
The update, called Service Pack 2, is intended to make the operating system from Microsoft much more secure.
The company, based in Redmond, US, said it needed extra time to ensure the proposed software changes were stable.
Analysts have described the upgrade as the most significant revision of Windows XP since its launch in 2001.
The update was originally due to be available in June.
It was then planned for release in July.
As well as offering bug fixes and updates, SP2 also has new features and makes significant changes to the software in the areas of network protection, memory protection, e-mail security and browsing security.
CHANGES DUE IN SP2
Pop-up ads blocked
Revamped firewall on by default
Outlook Express, Internet Explorer and Windows Messenger to carry warning about attachments
Origins of downloaded files logged
Web graphics in e-mail no longer loaded by default
Some spyware blocked
Users regularly reminded about Windows updates
Windows Messenger Service turned off by default
The download, which will be available free over the internet, will be a substantial size. The SP2 pack for home users will be about 70MB, while for business users the file will be about 90MB, said Mike Nash, Microsoft's vice-president of security.
The update service includes downloading technology that will allow users to download the file bit by bit, a feature that would be useful for those with dial-up internet connections.
Windows XP is the latest version of Windows. Microsoft operating systems are installed on more than 90% of the world's PCs.
The update has already been tested by thousands of developers and IT professionals and Microsoft says it will be its most widely tested product to date.
However, it warned that Windows XP SP2 could break existing applications.
The update is focusing on security which Microsoft has made a top priority since major worms such as Blaster and MyDoom which exploited flaws in Windows.