Sometimes you can have a portable device which is incompatible with Windows Media Player 11. So, the first step how to uninstall WMP11 and roll back actions to have WMP 10 again is via the Control Panel and Add/Remove Programs as on picture below:



But sometimes you cannot find this WMP 11 item in Add/Remove programs. So what to do now?
This:

Inspecting problem - no WMP11 uninstall in Add/Remove progs.
Removing WMP from Add/Remove Windows Components - didn't solve problem.
Restarting Windows, running uninstall from C:\WINDOWS\$NtUninstallwmp11$
Running uninstall from C:\WINDOWS\$NtUninstallWMFDist11$

Now installing WMP 10 - successful

PS: Beware of Windows update for WMP 11!