Rebooting loops.Turn off power, restart , keep tapping F8 or del button ( sometimes it's F2) and try select last known good configuration.If that doesn't work you need to repeat the above sequence but choose safe mode with networking.then try and run either combofix which you download from bleepingcomputer or malwarebytes to remove rootkit virus which usually found in boot sectors.You can run either within safe mode once you have a running machine look into putting ERDNt on your machine as long as it's not 64 bit it's a small program that is free and much better than the windows restore method as it saves a working copy of the registry every time you shut down so in the event of something like this it will go back to any day you choose and reset the registry of a working environment without losing any files or data.rootkit removal is quite difficult but if you google it you will find the best solution to your machine if you can't do any of the above.