Microsoft has fixed a bug in a critical security patch that has been causing problems for some users.

The bug is in the critical MS06-040 update, released last week, and affects programs that use up large chunks of memory on some versions of Windows. According to Microsoft, programs such as Navision 3.7, which require allocations of more than 1GB of memory, can crash after the update is installed.

Most Windows systems do not experience the bug, but Microsoft Windows Server 2003 and the 64-bit version of Windows XP Professional Edition are affected. Microsoft has produced a hotfix.

The majority of widely used applications allocate memory in chunks that are smaller than 1GB, so there have not been widespread reports of problems with this patch, according to Johannes Ullrich, chief research officer for the SANS Institute.

More troublesome has been the MS06-042 update for Internet Explorer, which has caused browser crashes while using Web-based applications such as PeopleSoft, Siebel and Unicenter. Microsoft issued a hotfix for that update earlier in the week and is promising to reissue the buggy update next Tuesday.

Microsoft issued a total of 12 updates this month, fixing 23 vulnerabilities.