The frustration that sets in when you think you're being subjected to a malware attack is no fun. It's hardly better when you realize that the source of the out-of-blue, pop-up advertisement is not malware, but a program like Start Menu 8 (a start menu replacement for Windows 8.x) that you may have had on your system for months or years. Iobit's loss-leader for its Advanced System Care product started popping up ads in the last couple of months, which in some circumstances, can stop the Start Menu 8 program from running during the first reboot after installing Start Menu 8.
There's nothing harmful about the ads, and the program will return to normal operation after the pop-up disappears. The company seemed a bit at sea about the cause but promised a fix soon.