Do you really mean "reboot" and "without a disk"?
Guessing that you actually mean reinstall without a copy of Windows, then unless the model of PC has a recovery partition on the hard disk (some do some don't it will be in the on-line manual), you can't.
"Reboot" is what the PC does every time that you turn it on. That is say it load the Operating System - which is what reboot means.
T, March 2011
