"Parallels tools are not installed" keeps appearing

At the bottom of my screen, I get this message every time I start:

Is there a problem? I reinstall, and I still get it.

 

Hello,

did you try removing Tools manually from Start -> Control Panel -> Add/Remove Programs, reboot and try to install Tools again?

Also, are you using Boot Camp or a virtual disk based machine?

 

This worked.
It seems that switching on and off hypervisor will trigger this error.