Microsoft has discovered after a in-depth investigation that a rootkit called Alureon was the cause of the reboot loop problem after installing several patches.

Rootkits are programs that replace system files to prevent an antivirus discovering what they are doing as well as provide trojan horses a chance to run programs without the users knowledge. A rootkit can cause a computer to send out spam emails.

Rootkits can be very difficult to discover and remove and in some cases it is easier reinstalling the computer from the CD than to remove the rootkit as too many program components may be damaged.