' is this blasted problem only affecting PCs upgraded to Win10 from Win7/8 or is it affecting new Win10 PCs? If it is, I'll be hacked off having just bought a new Win10 laptop. "Not fit for purpose"...

This seems to be a very common problem - nearly everyone I know with Windows 10 has experienced it at least once.

The solution I've always found works:

Use Safe Mode. You probably can't enter...