could be anything.
A while back ago pc's would loop like that if you had Nero and Roxio software on your pc at the same time. It could be that your running more than one antivirus/firewall too.
Type in "msconfig" without the quotes in your desktop search, open that to the start tab, disable each item in the start one at a time and reboot to see if it loops, Take note of each program and the program you disable when it stops is the program causing the issue.
Running antimalware is a good idea too, download, update and do a full scan of your pc with Malwarebytes