Users of two of Microsoft's most popular software packages, Excel and Powerpoint, have been warned to download a new security patch to prevent the return of macro viruses.

This type of virus can transfer between different documents by using the underlying macro programming language contained within Excel and Powerpoint.

Before the introduction of Office 2000, these types of macro viruses were very common and were often written by virus creators to corrupt electronic data.

But when it introduced Office 2000, Microsoft thought it had plugged the gap in the software that enabled the macro virus to transfer between documents.

It told computer-users when a macro was within a background document by causing a pop-up box to appear, according to Mr Andy Harbison, a Dublin-based consultant with Ernst & Young.

"Therefore, users could avoid the risk by simply turning the macros off," he added.

However, in the past few weeks a new bug has been identified in the Microsoft software that may enable macros to run without warning.

The bug was located by a security consultant working with US security firm Symantec. The company has classified the bug as "high-risk".

Symantec has warned that unauthorised macro files, potentially containing malicious code, could run without warning, successfully bypassing Microsoft's security features.

"It is now possible for a skilled person to write a virus to take advantage of this problem," said Mr Harbison.

Microsoft, which learned about the bug in its software two weeks ago, has made a software patch freely available on the security pages of its website at www.microsoft.com.

Meanwhile, the Sircam worm, which affected at least 30 firms in the Republic last month, may reappear next week.

This worm propagates by attaching itself to a computer file and e-mailing itself, and the file, to other computer users, using Outlook Express software.

The worm is particularly nasty because it changes the tag line on the e-mail, making it difficult to identify. It can also delete files and folders, and send confidential documents to other users.

According to security experts, there is a one-in-20 chance that Sircam will occur next Tuesday, October 16th, and delete files and folders stored on affected users' C drive.

A fix for the problem is available at www.sarc.com/avcentre /fix sirc.com.