Ok, I found the solution.
If anyone else is having the same issue, just run regedit, find this:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4d36e968-e325-11ce-bfc1-08002be10318}\0000
Then, find Display1_RotationCaps, and set the value to 7.
Rotation will be now available through the Screen Resolution menu by right clicking on the desktop.
Let's hear a big clap for the geniuses at HP (or whatever) that "hid" the very basic screen rotation feature in regedit for common users to accidentally f**k up their systems by trying to enable it.
Well done. We are very proud of you.
Keep it up wasting our time in such a magnificent manner.