Intermittant problem

Every once in a while, when I boot up, explorer.exe fails to start. I can Ctrl/Alt/Del to open “Task Manager” and start it from there and everything is fine. I’ve thoroughly scanned for spyware/viruses. This might not happen for 2 weeks and then happen every day for 3-4 days and stop again for an extended period. Can’t figure it out, no new updates in a month, no new software, no new printers. Anyone ever see this?