Microsoft's latest batch of patches contains a kernel update designed not to install on machines infected with a rootkit.

The move is designed to prevent the confusion that occurred when one of the patches released in February resulted in a Blue Screen of Death and continuous reboot cycles on some Windows XP machines.

Microsoft copped a fair bit of criticism for the incident before the cause was pinned down to the interaction between the hard-to-detect Tdss rootkit and a Windows kernel security update. Rootkits are a type of malware that attempt to avoid detection by anti-virus scanners by burying themselves in the likes of Windows kernel code.

Redmond's April patch batch also contains a Windows kernel patch. In an effort to prevent the same snafu as February, Microsoft is using technology designed to prevent the update from installing onto malware-compromised machines.

"This security update includes package-detection logic that prevents the installation of the security update if certain abnormal conditions exist" Microsoft explains in the FAQ for its MS10-21 patch.

"These abnormal conditions on a system could be the result of an infection with a computer virus that modifies some operating system files, which renders the infected computer incompatible with the kernel update."

The same technology was used in a modified version of the MS10-15 update, which was initially released as part of February's Patch Tuesday and revised with an "avoid infected systems" add-on in mid-March.

Users who attempt to install either the revised MS010-15 package or the new Ms10-021 kernel update on infected machines ought to get an error code.

Possible error codes (such as 0x8007F0F4) fail to explain what might have gone wrong, but a bit of judicious searching ought to direct sys admins or regular users who hit the snag to this advisory here. This notice explains that users who run across problems installing the patch are probably dealing with a malware-infected machine.

Microsoft's April patch batch included 11 bulletins along with an update to the software giant's Malicious Software Removal Tool. The idea is that this tool will remove malware and clean up systems which can then by safely patched at the second time of asking. ®