Microsoft has eliminated four of eight previously announced security patches scheduled to be available to system administrators tomorrow.

The software giant amended a notice posted on its website the day before announcing the scheduled eight patches.

Three of the bulletins removed addressed vulnerabilities in Windows, the Office business software suite and Visual Studio, a software development program, with a maximum severity rating of 'important'. The fourth one addressed a vulnerability in Windows with a maximum severity rating of 'critical'.

Microsoft said it still plans to release the remaining four patches. Three of them affect Office and one affects Windows; each is rated 'critical'.

'Critical' is the highest level of urgency on Microsoft's ranking system, indicating that the vulnerabilities could be exploited to run malicious software on a PC with no action on the part of the victim.

Asked for an explanation of the sudden change, a company spokesman only said: "There are many factors that impact the release of a security update, and every vulnerability presents its own unique challenges."

Microsoft releases security patches on the second Tuesday of each month, an event that has become known as Patch Tuesday to system administrators.