This paper presents ShadowReboot, a virtual machine monitor (VMM)-based approach that shortens the downtime for software updates during an OS reboot. ShadowReboot reboots the guest OS in the background by spawning a VM dedicated to an OS reboot and enables the user to switch over to the rebooted state where the updated kernel and applications are ready for use. ShadowReboot provides an illusion to the users that the guest OS travels forward in time to the rebooted state where the updated kernel and applications are ready for use. ShadowReboot offers the following advantages. It can be applied to any patch to the kernels and even system configuration updates. Second, it does not need any special patch requiring intimate knowledge about the target kernels. Third, it does not require any target kernel modification. We implemented a prototype in VirtualBox 3.0.8 OSE. Our preliminary experimental results show that ShadowReboot shortened the downtime of commodity OS reboots on Windows XP and five Linux distributions (Gentoo, Fedora, Cent, Ubuntu, and SUSE) by 43 to 96%.