Windows 8 auffrischen nicht möglich, weil Dateien fehlen / Windows 8 refresh fails because of missing files / Install Windows 8 with Product key only

A friend's Laptop went Kind of crazy, Adobe Flash was crashing all the time ("Shockwave Flash ist möglicherweise beschäftigt oder reagiert nicht mehr") and Skype refused to logon. I decided to do a "Windows refresh", but this failed because "some files are missing". We did not have an Windows DVD by hand, but we found a way to get it done.

How I fixed it:
You Need your Windows 8 Product Key for this!
Go to: http://windows.microsoft.com/de-DE/windows-8/upgrade-product-key-only

Choose the desired vesion of Windows 8 / 8.1, download and start the installation. This will refresh your system. After the Installation there will be many, many updates (106 for me) to apply!