I've had this before - very annoying. scanreg disappeared ofor a while but now it works perfectly.
Press Ctrl+R or open the run dialog box. Then type services.msc and see itf it comes up.
If it does, that's a workaround for the moment, but if not, check to see that no programs you've installed block it. (Check on the website)
Sometimes antivirus, firewall, or cleaning software can block it and replace it with something in their frontend.