Is there a way to minimize computer usage except for one program?

Solution 1:

Windows is quite complicated, and has numerous maintenance processes. If the intended use is just one application, and if it runs well in another OS, e.g. Windows RE (WinRE), or Lubuntu under wine, you may find the PC uses less energy with the alternative OS than under a full Windows version.

Since either WinRE or Lubuntu can be run from USB without needing any installation to HDD or SDD, you could test these alternatives on your specific PC, comparing their power usage, and that from Windows 7, with a watt meter.

However, the PC architecture may be the biggest obstacle to efficiency. Many newer PC's use far less power than older ones. so why not move to a dedicated device such as the Raspeberry Pi 3B, which about 1.3 watts idling, and about 2 watts full bore?