Microsoft has pulled the plug on a Windows update that snuck Skype onto business PCs.

Corporate admins got a nasty surprise on Wednesday when Skype 5.9 was automatically and silently installed on work machines via Windows Server Update Service (WSUS) - including PCs that did not have the voice chat software previously installed.

IT bods on this microsoft.com forum complained of hundreds of computers infected by the update and scrambled to remove the VoIP client centrally from PCs.

One admin blasted:

I administer several banks that belong to a holding company. I had to dispatch techs immediately to remove the software from appx 25 machines first thing this morning because they are in the middle of an IT audit and Skype is definitely not going to pass.

Another wrote:

I have hundreds of computers this morning with Skype 5.9 installed because WSUS pushed the update to the clients. WSUS will install Skype 5.9 on clients that have never had Skype installed if install Important Updates is enabled on the client.

Microsoft blessed this command-line magic to remove the unwanted Skype from machines, while also “expiring” the update – meaning it has been withdrawn.

The Windows software giant bought loss-making Skype in May 2011 for $8.5bn. In May Microsoft delivered tools for OEMs to install the VoIP client on consumers' PCs. ®