"Microsoft is telling Windows users that they'll have to reinstall the operating system if they get infected with a new rootkit that hides in the machine's boot sector.

A new variant of a Trojan Microsoft calls "Popureb" digs so deeply into the system that the only way to eradicate it is to return Windows to its out-of-the-box configuration, Chun Feng, an engineer with the Microsoft Malware Protection Center (MMPC), said last week on the group's blog.

"If your system does get infected with Trojan:Win32/Popureb.E, we advise you to fix the MBR and then use a recovery CD to restore your system to a pre-infected state," said Feng.

A recovery disc returns Windows to its factory settings."