I am running FSX with SP1 and SP2 (not Accel). No add-ons.
In the stock Cessna 172 SP, the radio stack consists of King KX 155As for Comm 1 and Comm2. You should be able to set the 25 kHz increment when selecting a frequency by pulling out on the frequency selection knob. The panel is placarded correctly under the knob to do that. However, there is no reaction when I mouse over the knob.
There appears to be no way to select the 25 kHz increment. This seems like a pretty severe limitation since so many facilities have those frequencies assigned for Tower, Approach Control, ATIS, etc. An example is Ramona, CA, USA, a strictly General Aviation field, where the Tower is 119.875 and ATIS is 132.025.
How do you set these frequencies?
I don't believe that you can. If you tune in the frequencies given within the program, they should work just fine. Just tune in 132.02 and the ATIS of 132.025 should work.