Basically it shouldn't keep turning off unless it's encountering malware (check the last link in my signature for some hints) or your operating system isn't being kept up to date, so make sure of that and, as McAfee interface uses Internet explorer regardless of your default browser settings, make sure you have the latest version of that installed.
You can always re-download your product from your online account: https://home.mcafee.com/secure/protected/login.aspx?rfhs=1
If you have any problems Technical Support is free of charge and always available by phone or online chat and linked under Useful Links at the top of this page.
Also ensure you have uninstalled all bits of the program before reinstalling The correct way to uninstall is as shown here......
reboot and run
and reinstall from your account as Peter mentioned.