Prior to your reply I removed all traces of FSUIPC4. Game not fixed. Within Steam I verified integrity of game files which were OK. Reinstalled FSX_SE. Still not fixed. Cleaned registry. Still not fixed so gave up, waiting your reply.
Re-ran FSUIPC4 installer (with admin rights). Appeared to run OK, but after searching for FSUIPC4.ini on C drive, it COULD NOT BE FOUND.
Tried launching FSX. OK'ed security warning about FSUIPC4.dll. Back to stuck splash screen and 55% CPU usage.
Note I've used FSX for over 250 hrs with no problems. The first install of FSUIPC4 without validation worked, and FSUIPC was visible in the FSX pull-down menu. It was only after the second install of FSUIPC4 with validation that the problem occurred.
PS. During the first working install I went to FSX Settings-Controls and unchecked 'Enable Controller(s)'. could that have caused a problem?