Hello and welcome to Tom's Hardware Forums. The Installer can go AWOL in Safe Mode but if you aren't in that, go from Start to Run and into the open box type services.msc then hit Enter. In the alphabetical list, highlight Windows installer and double click it.
In the Startup type box which probably says Disabled, select Automatic and click Apply. After that, click on Start Service and click Apply again then OK your way out. Restart the computer and it should be back to normal.