Have you updated anything recently?
What anti-malware programs are you running?
Have you run your anti-malware programs to ensure you are `clean`? It is best to run these in `Safe Mode` as they generally cannot delete `active` malware.
Another option is to go to `Start/Run/` and Type `msconfig`. Press enter.
Click on `Startup`
Click on the `Disable All` button, `Apply` then restart. When you boot again, try restarting. The problem should be gone.
What this does is remove all the software that is starting up with your computer. You can re-enabled it later as you troubleshoot what the cause is.
If the problem is still there, then you may have a problem with Windows itself.