Problem is, when I turn my computer on, the wheel won't calibrate itself right. It will do like half a rotation and stay off centered by ~20°. Only way to get it right is unplugging the USB for half a minute. Is there maybe a way to force a calibration via some sort of key combination?

Sometimes (seldom) It does the same also here, on WinXP.
I just unplug & replug it right away...

I get a slightly different problem where after start up the wheel always goes to about 300 degrees, even if it's set to 720 both in-game and in the control panel. So before use I go to the control panel and move the slider away from 720 degrees then back to it again to get it to work. Rather annoying to have to do this every time.

My wheel is stuck at 90 degrees, it won't recalibrate even i unplugged on startup and replugged when fully loaded.
And it fails calibrating it while the computer is starting, help?

I found out that if I turn the wheel after I shut down the PC, this wheel rotation is somewhat taken as the rotation it had when I shut down, therefor it wants to correct a wrong angle and the wheel ends up off center. :detective

Quote from Tomba(FIN) :

My wheel is stuck at 90 degrees, it won't recalibrate even i unplugged on startup and replugged when fully loaded.
And it fails calibrating it while the computer is starting, help?

Go into Logitech Profiler, go to the Device settings thing where it says degrees, just click ok on the box, without changing anything. It should work.