|It comes back from one of two reasons. One is you never got it out. Another is you have some infection locally or on network or on usb drive that keeps putting it back on.|
The main problem is you were running it in an un-secure manner. You may have to reload from OEM disks and then learn and use as many best practices as you can to avoid that in the future. A well protected system is less likely to get this issue.
1/3 of highway deaths are caused by drunks. The rest are by people who can't drive any better than a drunk.