Why does enabling XMP make my new PC crash in games?

This is probably memory instability rather than Windows. You can try lowering the frequency, loosening the timings, or slightly increasing DRAM voltage. Some kits also include a lower-speed profile that may be easier to run. If you do not want to manually tune the memory, exchanging the kit could be simpler. Run a proper memory stability test after every change.

On some B760 systems, XMP becomes more reliable if DRAM voltage and DRAM training voltage are set manually to match the XMP value. Disabling memory fast boot can also force proper memory training on each startup. It is worth checking that the CPU cooler is not overtightened, since excessive mounting pressure on LGA1700 can occasionally cause memory-channel problems.

Try setting the DRAM voltage manually to around 1.37–1.40V, but check the memory IC version first because not every kit should be pushed to the same voltage. Also make sure the sticks are installed in the recommended dual-channel slots, usually A2 and B2—the second and fourth slots from the CPU.

Running the memory at a lower speed is a perfectly reasonable workaround if the system is stable that way. In real-world gaming, the performance difference between the advertised speed and something like 2666–2933MHz may be small, especially if stability is the priority. A clean Windows installation is unlikely to fix crashes caused specifically by enabling XMP, though reinstalling may still be worthwhile for general system cleanup.