OK, I've taken the risk and it was worth it - everything works normally, but I have freed the 17GB on my SSD now.
Basically, the steps are:
- Restart your PC, press F8 continuously to bring up boot menu, and choose "Command prompt with safe mode" (this is to ensure you can move the windows folder around)
- Once the command prompt is shown, type the command to move the folder to another drive:
robocopy C:\Windows\Installer D:\Windows\Installer /MOVE /e
- Next, create a "Junction" link for the missing folder:
mklink /J C:\Windows\Installer D:\Windows\Installer
Of course, this assumes that your %WINDIR% is C:\Windows, that your D drive is the one with more space, that you want your destination in "D:\Windows\Installer" (it could be any folder, just be consistent) etc...
This should do the trick.